diff --git a/.kiro/specs/ultimate-lists-app/tasks.md b/.kiro/specs/ultimate-lists-app/tasks.md
index 24bd1d5..153e227 100644
--- a/.kiro/specs/ultimate-lists-app/tasks.md
+++ b/.kiro/specs/ultimate-lists-app/tasks.md
@@ -199,7 +199,7 @@
   - Implement skeleton loaders
   - _Requirements: 7.3_
 
-- [-] 16. Implement error handling
+- [x] 16. Implement error handling
   - Add try-catch blocks to all server actions
   - Implement error boundaries in React components
   - Add user-friendly error messages
@@ -207,7 +207,7 @@
   - Implement retry logic for failed operations
   - _Requirements: All (error handling is cross-cutting)_
 
-- [ ] 17. Add input validation
+- [x] 17. Add input validation
   - Validate all form inputs before submission
   - Add inline validation errors
   - Prevent empty task/list creation
diff --git a/components/InlineItemCreate.tsx b/components/InlineItemCreate.tsx
index bf757a5..111cae7 100644
--- a/components/InlineItemCreate.tsx
+++ b/components/InlineItemCreate.tsx
@@ -5,6 +5,7 @@ import { useRouter } from 'next/navigation'
 import { createItem } from '@/lib/actions'
 import { Plus } from 'lucide-react'
 import { useToast } from './ToastContainer'
+import { validateItemTitle, sanitizeString } from '@/lib/validation'
 
 interface InlineItemCreateProps {
   listId: string
@@ -16,25 +17,36 @@ export default function InlineItemCreate({ listId }: InlineItemCreateProps) {
   const [isCreating, setIsCreating] = useState(false)
   const [title, setTitle] = useState('')
   const [isSubmitting, setIsSubmitting] = useState(false)
+  const [error, setError] = useState('')
 
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault()
     
-    if (!title.trim() || isSubmitting) {
+    // Validate title
+    const validationResult = validateItemTitle(title)
+    if (!validationResult.isValid) {
+      setError(validationResult.error || 'Invalid title')
+      return
+    }
+
+    if (isSubmitting) {
       return
     }
 
     setIsSubmitting(true)
+    setError('')
 
     try {
-      await createItem(listId, title.trim())
+      await createItem(listId, sanitizeString(title))
       showToast('Task added successfully', 'success')
       setTitle('')
       setIsCreating(false)
       router.refresh()
     } catch (error) {
       console.error('Error creating item:', error)
-      showToast('Failed to add task', 'error')
+      const errorMessage = error instanceof Error ? error.message : 'Failed to add task'
+      setError(errorMessage)
+      showToast(errorMessage, 'error')
     } finally {
       setIsSubmitting(false)
     }
@@ -65,15 +77,30 @@ export default function InlineItemCreate({ listId }: InlineItemCreateProps) {
 
   return (
     <form onSubmit={handleSubmit} className="bg-white border-2 border-blue-400 rounded-lg p-4 sm:p-5 shadow-sm">
-      <input
-        type="text"
-        value={title}
-        onChange={(e) => setTitle(e.target.value)}
-        onKeyDown={handleKeyDown}
-        placeholder="Enter task title..."
-        autoFocus
-        className="w-full px-3 sm:px-4 py-3 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500 mb-3 sm:mb-4 text-sm sm:text-base"
-      />
+      <div className="mb-3 sm:mb-4">
+        <input
+          type="text"
+          value={title}
+          onChange={(e) => {
+            setTitle(e.target.value)
+            setError('')
+          }}
+          onKeyDown={handleKeyDown}
+          placeholder="Enter task title..."
+          autoFocus
+          maxLength={200}
+          className={`w-full px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 text-sm sm:text-base ${
+            error ? 'border-red-500 focus:ring-red-500' : 'border-gray-300 focus:ring-blue-500'
+          }`}
+          aria-invalid={error ? 'true' : 'false'}
+          aria-describedby={error ? 'inline-title-error' : undefined}
+        />
+        {error && (
+          <p id="inline-title-error" className="mt-1.5 text-xs sm:text-sm text-red-600">
+            {error}
+          </p>
+        )}
+      </div>
       <div className="flex flex-col-reverse sm:flex-row gap-2 sm:gap-3">
         <button
           type="button"
diff --git a/components/ItemForm.tsx b/components/ItemForm.tsx
index 5bc24df..1fbaf32 100644
--- a/components/ItemForm.tsx
+++ b/components/ItemForm.tsx
@@ -1,9 +1,18 @@
 'use client'
 
-import { useState } from 'react'
+import { useState, useEffect } from 'react'
 import { useRouter } from 'next/navigation'
 import { updateItem, createItem } from '@/lib/actions'
 import { useToast } from './ToastContainer'
+import {
+  validateItemTitle,
+  validateDescription,
+  validateDateString,
+  validateTag,
+  validateTags,
+  validateNotes,
+  sanitizeString,
+} from '@/lib/validation'
 
 interface ItemFormProps {
   listId: string
@@ -24,6 +33,7 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
   const { showToast } = useToast()
   const [isSubmitting, setIsSubmitting] = useState(false)
   const [errors, setErrors] = useState<Record<string, string>>({})
+  const [touched, setTouched] = useState<Record<string, boolean>>({})
 
   // Form state
   const [title, setTitle] = useState(item?.title || '')
@@ -40,18 +50,107 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
   const [tagInput, setTagInput] = useState('')
   const [notes, setNotes] = useState(item?.notes || '')
 
+  // Inline validation for title
+  useEffect(() => {
+    if (touched.title) {
+      const result = validateItemTitle(title)
+      setErrors((prev) => ({
+        ...prev,
+        title: result.isValid ? '' : result.error || '',
+      }))
+    }
+  }, [title, touched.title])
+
+  // Inline validation for description
+  useEffect(() => {
+    if (touched.description) {
+      const result = validateDescription(description)
+      setErrors((prev) => ({
+        ...prev,
+        description: result.isValid ? '' : result.error || '',
+      }))
+    }
+  }, [description, touched.description])
+
+  // Inline validation for due date
+  useEffect(() => {
+    if (touched.dueDate && dueDate) {
+      const result = validateDateString(dueDate)
+      setErrors((prev) => ({
+        ...prev,
+        dueDate: result.isValid ? '' : result.error || '',
+      }))
+    } else if (touched.dueDate && !dueDate) {
+      setErrors((prev) => ({ ...prev, dueDate: '' }))
+    }
+  }, [dueDate, touched.dueDate])
+
+  // Inline validation for notes
+  useEffect(() => {
+    if (touched.notes) {
+      const result = validateNotes(notes)
+      setErrors((prev) => ({
+        ...prev,
+        notes: result.isValid ? '' : result.error || '',
+      }))
+    }
+  }, [notes, touched.notes])
+
+  // Validate tags whenever they change
+  useEffect(() => {
+    const result = validateTags(tags)
+    setErrors((prev) => ({
+      ...prev,
+      tags: result.isValid ? '' : result.error || '',
+    }))
+  }, [tags])
+
+  const handleBlur = (field: string) => {
+    setTouched((prev) => ({ ...prev, [field]: true }))
+  }
+
   const validateForm = () => {
     const newErrors: Record<string, string> = {}
 
-    if (!title.trim()) {
-      newErrors.title = 'Title is required'
+    // Validate title
+    const titleResult = validateItemTitle(title)
+    if (!titleResult.isValid) {
+      newErrors.title = titleResult.error || ''
     }
 
-    if (title.length > 200) {
-      newErrors.title = 'Title must be less than 200 characters'
+    // Validate description
+    const descResult = validateDescription(description)
+    if (!descResult.isValid) {
+      newErrors.description = descResult.error || ''
+    }
+
+    // Validate due date
+    if (dueDate) {
+      const dateResult = validateDateString(dueDate)
+      if (!dateResult.isValid) {
+        newErrors.dueDate = dateResult.error || ''
+      }
+    }
+
+    // Validate tags
+    const tagsResult = validateTags(tags)
+    if (!tagsResult.isValid) {
+      newErrors.tags = tagsResult.error || ''
+    }
+
+    // Validate notes
+    const notesResult = validateNotes(notes)
+    if (!notesResult.isValid) {
+      newErrors.notes = notesResult.error || ''
     }
 
     setErrors(newErrors)
+    setTouched({
+      title: true,
+      description: true,
+      dueDate: true,
+      notes: true,
+    })
     return Object.keys(newErrors).length === 0
   }
 
@@ -59,6 +158,7 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
     e.preventDefault()
 
     if (!validateForm()) {
+      showToast('Please fix validation errors', 'error')
       return
     }
 
@@ -66,12 +166,12 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
 
     try {
       const data = {
-        title: title.trim(),
-        description: description.trim() || null,
+        title: sanitizeString(title),
+        description: sanitizeString(description) || null,
         priority,
         dueDate: dueDate ? new Date(dueDate) : null,
-        tags,
-        notes: notes.trim() || null,
+        tags: tags.map(sanitizeString),
+        notes: sanitizeString(notes) || null,
       }
 
       if (mode === 'create') {
@@ -90,19 +190,39 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
       }
     } catch (error) {
       console.error('Error saving item:', error)
-      setErrors({ submit: 'Failed to save item. Please try again.' })
-      showToast('Failed to save task', 'error')
+      const errorMessage = error instanceof Error ? error.message : 'Failed to save item. Please try again.'
+      setErrors({ submit: errorMessage })
+      showToast(errorMessage, 'error')
     } finally {
       setIsSubmitting(false)
     }
   }
 
   const handleAddTag = () => {
-    const tag = tagInput.trim()
-    if (tag && !tags.includes(tag)) {
-      setTags([...tags, tag])
-      setTagInput('')
+    const tag = sanitizeString(tagInput)
+    
+    // Validate tag
+    const tagResult = validateTag(tag)
+    if (!tagResult.isValid) {
+      setErrors((prev) => ({ ...prev, tagInput: tagResult.error || '' }))
+      return
     }
+    
+    // Check for duplicates
+    if (tags.includes(tag)) {
+      setErrors((prev) => ({ ...prev, tagInput: 'Tag already exists' }))
+      return
+    }
+    
+    // Check max tags
+    if (tags.length >= 20) {
+      setErrors((prev) => ({ ...prev, tagInput: 'Maximum 20 tags allowed' }))
+      return
+    }
+    
+    setTags([...tags, tag])
+    setTagInput('')
+    setErrors((prev) => ({ ...prev, tagInput: '' }))
   }
 
   const handleRemoveTag = (tagToRemove: string) => {
@@ -128,13 +248,21 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
           id="title"
           value={title}
           onChange={(e) => setTitle(e.target.value)}
-          className={`w-full px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500 text-sm sm:text-base ${
-            errors.title ? 'border-red-500' : 'border-gray-300'
+          onBlur={() => handleBlur('title')}
+          className={`w-full px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 text-sm sm:text-base ${
+            errors.title && touched.title
+              ? 'border-red-500 focus:ring-red-500'
+              : 'border-gray-300 focus:ring-blue-500'
           }`}
           placeholder="Enter task title"
+          maxLength={200}
+          aria-invalid={errors.title && touched.title ? 'true' : 'false'}
+          aria-describedby={errors.title && touched.title ? 'title-error' : undefined}
         />
-        {errors.title && (
-          <p className="mt-1.5 text-xs sm:text-sm text-red-600">{errors.title}</p>
+        {errors.title && touched.title && (
+          <p id="title-error" className="mt-1.5 text-xs sm:text-sm text-red-600">
+            {errors.title}
+          </p>
         )}
       </div>
 
@@ -147,10 +275,23 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
           id="description"
           value={description}
           onChange={(e) => setDescription(e.target.value)}
+          onBlur={() => handleBlur('description')}
           rows={3}
-          className="w-full px-3 sm:px-4 py-3 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500 text-sm sm:text-base resize-y"
+          className={`w-full px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 text-sm sm:text-base resize-y ${
+            errors.description && touched.description
+              ? 'border-red-500 focus:ring-red-500'
+              : 'border-gray-300 focus:ring-blue-500'
+          }`}
           placeholder="Add a description"
+          maxLength={2000}
+          aria-invalid={errors.description && touched.description ? 'true' : 'false'}
+          aria-describedby={errors.description && touched.description ? 'description-error' : undefined}
         />
+        {errors.description && touched.description && (
+          <p id="description-error" className="mt-1.5 text-xs sm:text-sm text-red-600">
+            {errors.description}
+          </p>
+        )}
       </div>
 
       {/* Priority */}
@@ -180,14 +321,28 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
           id="dueDate"
           value={dueDate}
           onChange={(e) => setDueDate(e.target.value)}
-          className="w-full px-3 sm:px-4 py-3 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500 text-sm sm:text-base tap-target"
+          onBlur={() => handleBlur('dueDate')}
+          className={`w-full px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 text-sm sm:text-base tap-target ${
+            errors.dueDate && touched.dueDate
+              ? 'border-red-500 focus:ring-red-500'
+              : 'border-gray-300 focus:ring-blue-500'
+          }`}
+          min="1900-01-01"
+          max="2100-12-31"
+          aria-invalid={errors.dueDate && touched.dueDate ? 'true' : 'false'}
+          aria-describedby={errors.dueDate && touched.dueDate ? 'dueDate-error' : undefined}
         />
+        {errors.dueDate && touched.dueDate && (
+          <p id="dueDate-error" className="mt-1.5 text-xs sm:text-sm text-red-600">
+            {errors.dueDate}
+          </p>
+        )}
       </div>
 
       {/* Tags */}
       <div>
         <label htmlFor="tags" className="block text-sm font-medium text-gray-700 mb-2">
-          Tags
+          Tags {tags.length > 0 && <span className="text-gray-500">({tags.length}/20)</span>}
         </label>
         <div className="flex gap-2 mb-2 sm:mb-3">
           <input
@@ -196,17 +351,33 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
             value={tagInput}
             onChange={(e) => setTagInput(e.target.value)}
             onKeyDown={handleKeyDown}
-            className="flex-1 px-3 sm:px-4 py-3 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500 text-sm sm:text-base"
+            className={`flex-1 px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 text-sm sm:text-base ${
+              errors.tagInput
+                ? 'border-red-500 focus:ring-red-500'
+                : 'border-gray-300 focus:ring-blue-500'
+            }`}
             placeholder="Add a tag"
+            maxLength={50}
+            aria-invalid={errors.tagInput ? 'true' : 'false'}
+            aria-describedby={errors.tagInput ? 'tagInput-error' : undefined}
           />
           <button
             type="button"
             onClick={handleAddTag}
-            className="px-4 sm:px-5 py-3 bg-gray-200 hover:bg-gray-300 active:bg-gray-400 text-gray-700 rounded-lg transition-colors tap-target text-sm sm:text-base font-medium"
+            disabled={tags.length >= 20}
+            className="px-4 sm:px-5 py-3 bg-gray-200 hover:bg-gray-300 active:bg-gray-400 disabled:bg-gray-100 disabled:text-gray-400 text-gray-700 rounded-lg transition-colors tap-target text-sm sm:text-base font-medium"
           >
             Add
           </button>
         </div>
+        {errors.tagInput && (
+          <p id="tagInput-error" className="mb-2 text-xs sm:text-sm text-red-600">
+            {errors.tagInput}
+          </p>
+        )}
+        {errors.tags && (
+          <p className="mb-2 text-xs sm:text-sm text-red-600">{errors.tags}</p>
+        )}
         {tags.length > 0 && (
           <div className="flex flex-wrap gap-2">
             {tags.map((tag) => (
@@ -238,10 +409,23 @@ export default function ItemForm({ listId, item, mode = 'edit' }: ItemFormProps)
           id="notes"
           value={notes}
           onChange={(e) => setNotes(e.target.value)}
+          onBlur={() => handleBlur('notes')}
           rows={5}
-          className="w-full px-3 sm:px-4 py-3 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500 text-sm sm:text-base resize-y"
+          className={`w-full px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 text-sm sm:text-base resize-y ${
+            errors.notes && touched.notes
+              ? 'border-red-500 focus:ring-red-500'
+              : 'border-gray-300 focus:ring-blue-500'
+          }`}
           placeholder="Add additional notes"
+          maxLength={5000}
+          aria-invalid={errors.notes && touched.notes ? 'true' : 'false'}
+          aria-describedby={errors.notes && touched.notes ? 'notes-error' : undefined}
         />
+        {errors.notes && touched.notes && (
+          <p id="notes-error" className="mt-1.5 text-xs sm:text-sm text-red-600">
+            {errors.notes}
+          </p>
+        )}
       </div>
 
       {/* Error message */}
diff --git a/components/ListForm.tsx b/components/ListForm.tsx
index d7ed548..9e36c1d 100644
--- a/components/ListForm.tsx
+++ b/components/ListForm.tsx
@@ -1,9 +1,10 @@
 'use client'
 
-import { useState } from 'react'
+import { useState, useEffect } from 'react'
 import { createList, updateList } from '@/lib/actions'
 import { useRouter } from 'next/navigation'
 import { useToast } from './ToastContainer'
+import { validateListTitle } from '@/lib/validation'
 
 interface ListFormProps {
   list?: {
@@ -33,12 +34,30 @@ export default function ListForm({ list, onCancel, onSuccess }: ListFormProps) {
   const [color, setColor] = useState(list?.color || PRESET_COLORS[0])
   const [isSubmitting, setIsSubmitting] = useState(false)
   const [error, setError] = useState('')
+  const [touched, setTouched] = useState(false)
+
+  // Inline validation when title changes
+  useEffect(() => {
+    if (touched) {
+      const result = validateListTitle(title)
+      setError(result.isValid ? '' : result.error || '')
+    }
+  }, [title, touched])
+
+  const handleTitleBlur = () => {
+    setTouched(true)
+    const result = validateListTitle(title)
+    setError(result.isValid ? '' : result.error || '')
+  }
 
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault()
     
-    if (!title.trim()) {
-      setError('List title is required')
+    // Validate before submission
+    const result = validateListTitle(title)
+    if (!result.isValid) {
+      setError(result.error || 'Invalid input')
+      setTouched(true)
       return
     }
 
@@ -62,8 +81,9 @@ export default function ListForm({ list, onCancel, onSuccess }: ListFormProps) {
         router.refresh()
       }
     } catch (err) {
-      setError('Failed to save list. Please try again.')
-      showToast('Failed to save list', 'error')
+      const errorMessage = err instanceof Error ? err.message : 'Failed to save list. Please try again.'
+      setError(errorMessage)
+      showToast(errorMessage, 'error')
       console.error(err)
     } finally {
       setIsSubmitting(false)
@@ -74,18 +94,31 @@ export default function ListForm({ list, onCancel, onSuccess }: ListFormProps) {
     <form onSubmit={handleSubmit} className="space-y-4 sm:space-y-5">
       <div>
         <label htmlFor="title" className="block text-sm font-medium text-gray-700 mb-2">
-          List Title
+          List Title *
         </label>
         <input
           type="text"
           id="title"
           value={title}
           onChange={(e) => setTitle(e.target.value)}
-          className="w-full px-3 sm:px-4 py-3 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-blue-500 text-sm sm:text-base"
+          onBlur={handleTitleBlur}
+          className={`w-full px-3 sm:px-4 py-3 border rounded-lg focus:outline-none focus:ring-2 text-sm sm:text-base ${
+            error && touched
+              ? 'border-red-500 focus:ring-red-500'
+              : 'border-gray-300 focus:ring-blue-500'
+          }`}
           placeholder="Enter list title"
           disabled={isSubmitting}
           autoFocus
+          maxLength={200}
+          aria-invalid={error && touched ? 'true' : 'false'}
+          aria-describedby={error && touched ? 'title-error' : undefined}
         />
+        {error && touched && (
+          <p id="title-error" className="mt-1.5 text-xs sm:text-sm text-red-600">
+            {error}
+          </p>
+        )}
       </div>
 
       <div>
@@ -111,10 +144,6 @@ export default function ListForm({ list, onCancel, onSuccess }: ListFormProps) {
         </div>
       </div>
 
-      {error && (
-        <div className="text-red-600 text-xs sm:text-sm font-medium">{error}</div>
-      )}
-
       <div className="flex flex-col-reverse sm:flex-row gap-2 sm:gap-3 justify-end pt-2">
         {onCancel && (
           <button
diff --git a/lib/actions.ts b/lib/actions.ts
index d5ed1b4..4435752 100644
--- a/lib/actions.ts
+++ b/lib/actions.ts
@@ -2,6 +2,14 @@
 
 import { prisma } from './db'
 import { revalidatePath } from 'next/cache'
+import {
+  validateListTitle,
+  validateItemTitle,
+  validateColor,
+  validateDateString,
+  validateTags,
+  sanitizeString,
+} from './validation'
 
 // Retry utility for database operations
 async function retryOperation<T>(
@@ -57,19 +65,23 @@ export async function getLists() {
 
 export async function createList(title: string, color: string = '#3B82F6') {
   try {
-    // Validate input
-    if (!title || !title.trim()) {
-      throw new Error('List title is required')
+    // Validate and sanitize input
+    const titleResult = validateListTitle(title)
+    if (!titleResult.isValid) {
+      throw new Error(titleResult.error)
     }
     
-    if (title.length > 200) {
-      throw new Error('List title must be less than 200 characters')
+    const colorResult = validateColor(color)
+    if (!colorResult.isValid) {
+      throw new Error(colorResult.error)
     }
     
+    const sanitizedTitle = sanitizeString(title)
+    
     const list = await retryOperation(() =>
       prisma.list.create({
         data: {
-          title: title.trim(),
+          title: sanitizedTitle,
           color,
         },
       })
@@ -79,7 +91,7 @@ export async function createList(title: string, color: string = '#3B82F6') {
     return list
   } catch (error) {
     console.error('Error creating list:', error)
-    if (error instanceof Error && error.message.includes('required')) {
+    if (error instanceof Error && (error.message.includes('required') || error.message.includes('must be'))) {
       throw error
     }
     throw new Error('Failed to create list. Please try again.')
@@ -165,24 +177,35 @@ export async function createItem(
       throw new Error('List ID is required')
     }
     
-    if (!title || !title.trim()) {
-      throw new Error('Item title is required')
+    const titleResult = validateItemTitle(title)
+    if (!titleResult.isValid) {
+      throw new Error(titleResult.error)
     }
     
-    if (title.length > 200) {
-      throw new Error('Item title must be less than 200 characters')
+    // Validate tags if provided
+    if (metadata?.tags) {
+      const tagsResult = validateTags(metadata.tags)
+      if (!tagsResult.isValid) {
+        throw new Error(tagsResult.error)
+      }
     }
     
+    // Sanitize inputs
+    const sanitizedTitle = sanitizeString(title)
+    const sanitizedDescription = description ? sanitizeString(description) : null
+    const sanitizedNotes = metadata?.notes ? sanitizeString(metadata.notes) : null
+    const sanitizedTags = metadata?.tags ? metadata.tags.map(sanitizeString) : undefined
+    
     const item = await retryOperation(() =>
       prisma.item.create({
         data: {
-          title: title.trim(),
-          description: description?.trim() || null,
+          title: sanitizedTitle,
+          description: sanitizedDescription,
           listId,
           ...(metadata?.priority && { priority: metadata.priority }),
           ...(metadata?.dueDate !== undefined && { dueDate: metadata.dueDate }),
-          ...(metadata?.tags && { tags: metadata.tags }),
-          ...(metadata?.notes !== undefined && { notes: metadata.notes }),
+          ...(sanitizedTags && { tags: sanitizedTags }),
+          ...(sanitizedNotes !== undefined && { notes: sanitizedNotes }),
         },
       })
     )
@@ -191,7 +214,7 @@ export async function createItem(
     return item
   } catch (error) {
     console.error('Error creating item:', error)
-    if (error instanceof Error && (error.message.includes('required') || error.message.includes('must be'))) {
+    if (error instanceof Error && (error.message.includes('required') || error.message.includes('must be') || error.message.includes('Invalid'))) {
       throw error
     }
     throw new Error('Failed to create item. Please try again.')
@@ -247,20 +270,32 @@ export async function updateItem(
       throw new Error('Item ID is required')
     }
     
-    // Validate title if provided
+    // Validate and sanitize title if provided
     if (data.title !== undefined) {
-      if (!data.title || !data.title.trim()) {
-        throw new Error('Item title cannot be empty')
+      const titleResult = validateItemTitle(data.title)
+      if (!titleResult.isValid) {
+        throw new Error(titleResult.error)
       }
-      if (data.title.length > 200) {
-        throw new Error('Item title must be less than 200 characters')
-      }
-      data.title = data.title.trim()
+      data.title = sanitizeString(data.title)
     }
     
-    // Trim description if provided
+    // Sanitize description if provided
     if (data.description !== undefined && data.description !== null) {
-      data.description = data.description.trim() || null
+      data.description = sanitizeString(data.description) || null
+    }
+    
+    // Validate tags if provided
+    if (data.tags !== undefined) {
+      const tagsResult = validateTags(data.tags)
+      if (!tagsResult.isValid) {
+        throw new Error(tagsResult.error)
+      }
+      data.tags = data.tags.map(sanitizeString)
+    }
+    
+    // Sanitize notes if provided
+    if (data.notes !== undefined && data.notes !== null) {
+      data.notes = sanitizeString(data.notes) || null
     }
     
     const item = await retryOperation(() =>
@@ -282,7 +317,7 @@ export async function updateItem(
     return updated
   } catch (error) {
     console.error('Error updating item:', error)
-    if (error instanceof Error && (error.message.includes('not found') || error.message.includes('required') || error.message.includes('cannot be'))) {
+    if (error instanceof Error && (error.message.includes('not found') || error.message.includes('required') || error.message.includes('cannot be') || error.message.includes('Invalid'))) {
       throw error
     }
     throw new Error('Failed to update item. Please try again.')
@@ -364,15 +399,21 @@ export async function updateList(
       throw new Error('List ID is required')
     }
     
-    // Validate title if provided
+    // Validate and sanitize title if provided
     if (data.title !== undefined) {
-      if (!data.title || !data.title.trim()) {
-        throw new Error('List title cannot be empty')
+      const titleResult = validateListTitle(data.title)
+      if (!titleResult.isValid) {
+        throw new Error(titleResult.error)
       }
-      if (data.title.length > 200) {
-        throw new Error('List title must be less than 200 characters')
+      data.title = sanitizeString(data.title)
+    }
+    
+    // Validate color if provided
+    if (data.color !== undefined) {
+      const colorResult = validateColor(data.color)
+      if (!colorResult.isValid) {
+        throw new Error(colorResult.error)
       }
-      data.title = data.title.trim()
     }
     
     const updated = await retryOperation(() =>
@@ -387,7 +428,7 @@ export async function updateList(
     return updated
   } catch (error) {
     console.error('Error updating list:', error)
-    if (error instanceof Error && (error.message.includes('not found') || error.message.includes('required') || error.message.includes('cannot be'))) {
+    if (error instanceof Error && (error.message.includes('not found') || error.message.includes('required') || error.message.includes('cannot be') || error.message.includes('Invalid'))) {
       throw error
     }
     throw new Error('Failed to update list. Please try again.')
diff --git a/lib/validation.ts b/lib/validation.ts
new file mode 100644
index 0000000..2c6a3f4
--- /dev/null
+++ b/lib/validation.ts
@@ -0,0 +1,303 @@
+/**
+ * Input validation utilities for the Ultimate Lists App
+ * Provides validation functions and sanitization for user inputs
+ */
+
+export interface ValidationResult {
+  isValid: boolean
+  error?: string
+}
+
+/**
+ * Sanitize string input by trimming whitespace
+ */
+export function sanitizeString(input: string | null | undefined): string {
+  if (input === null || input === undefined) {
+    return ''
+  }
+  return input.trim()
+}
+
+/**
+ * Validate list title
+ */
+export function validateListTitle(title: string): ValidationResult {
+  const sanitized = sanitizeString(title)
+  
+  if (!sanitized) {
+    return { isValid: false, error: 'List title is required' }
+  }
+  
+  if (sanitized.length > 200) {
+    return { isValid: false, error: 'List title must be less than 200 characters' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate item/task title
+ */
+export function validateItemTitle(title: string): ValidationResult {
+  const sanitized = sanitizeString(title)
+  
+  if (!sanitized) {
+    return { isValid: false, error: 'Task title is required' }
+  }
+  
+  if (sanitized.length > 200) {
+    return { isValid: false, error: 'Task title must be less than 200 characters' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate hex color format
+ */
+export function validateColor(color: string): ValidationResult {
+  const hexColorRegex = /^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/
+  
+  if (!hexColorRegex.test(color)) {
+    return { isValid: false, error: 'Invalid color format. Use hex format (e.g., #3B82F6)' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate priority value
+ */
+export function validatePriority(priority: string): ValidationResult {
+  const validPriorities = ['low', 'medium', 'high']
+  
+  if (!validPriorities.includes(priority)) {
+    return { isValid: false, error: 'Priority must be low, medium, or high' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate date string format (ISO 8601 or YYYY-MM-DD)
+ */
+export function validateDateString(dateString: string): ValidationResult {
+  if (!dateString) {
+    return { isValid: true } // Empty date is valid (optional field)
+  }
+  
+  // Try to parse the date
+  const date = new Date(dateString)
+  
+  // Check if date is valid
+  if (isNaN(date.getTime())) {
+    return { isValid: false, error: 'Invalid date format. Use YYYY-MM-DD format' }
+  }
+  
+  // Check if date is reasonable (not too far in past or future)
+  const minDate = new Date('1900-01-01')
+  const maxDate = new Date('2100-12-31')
+  
+  if (date < minDate || date > maxDate) {
+    return { isValid: false, error: 'Date must be between 1900 and 2100' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate tag string
+ */
+export function validateTag(tag: string): ValidationResult {
+  const sanitized = sanitizeString(tag)
+  
+  if (!sanitized) {
+    return { isValid: false, error: 'Tag cannot be empty' }
+  }
+  
+  if (sanitized.length > 50) {
+    return { isValid: false, error: 'Tag must be less than 50 characters' }
+  }
+  
+  // Disallow special characters that might cause issues
+  const invalidCharsRegex = /[<>\"']/
+  if (invalidCharsRegex.test(sanitized)) {
+    return { isValid: false, error: 'Tag contains invalid characters' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate array of tags
+ */
+export function validateTags(tags: string[]): ValidationResult {
+  if (!Array.isArray(tags)) {
+    return { isValid: false, error: 'Tags must be an array' }
+  }
+  
+  if (tags.length > 20) {
+    return { isValid: false, error: 'Maximum 20 tags allowed' }
+  }
+  
+  for (const tag of tags) {
+    const result = validateTag(tag)
+    if (!result.isValid) {
+      return result
+    }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate description text
+ */
+export function validateDescription(description: string | null | undefined): ValidationResult {
+  if (!description) {
+    return { isValid: true } // Description is optional
+  }
+  
+  if (description.length > 2000) {
+    return { isValid: false, error: 'Description must be less than 2000 characters' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate notes text
+ */
+export function validateNotes(notes: string | null | undefined): ValidationResult {
+  if (!notes) {
+    return { isValid: true } // Notes are optional
+  }
+  
+  if (notes.length > 5000) {
+    return { isValid: false, error: 'Notes must be less than 5000 characters' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Sanitize HTML to prevent XSS attacks
+ * Basic implementation - strips HTML tags
+ */
+export function sanitizeHtml(input: string): string {
+  return input.replace(/<[^>]*>/g, '')
+}
+
+/**
+ * Validate and sanitize list creation data
+ */
+export function validateListData(data: { title: string; color?: string }): {
+  isValid: boolean
+  errors: Record<string, string>
+  sanitized: { title: string; color: string }
+} {
+  const errors: Record<string, string> = {}
+  
+  // Validate title
+  const titleResult = validateListTitle(data.title)
+  if (!titleResult.isValid) {
+    errors.title = titleResult.error!
+  }
+  
+  // Validate color if provided
+  const color = data.color || '#3B82F6'
+  const colorResult = validateColor(color)
+  if (!colorResult.isValid) {
+    errors.color = colorResult.error!
+  }
+  
+  return {
+    isValid: Object.keys(errors).length === 0,
+    errors,
+    sanitized: {
+      title: sanitizeString(data.title),
+      color,
+    },
+  }
+}
+
+/**
+ * Validate and sanitize item creation data
+ */
+export function validateItemData(data: {
+  title: string
+  description?: string | null
+  priority?: string
+  dueDate?: string | null
+  tags?: string[]
+  notes?: string | null
+}): {
+  isValid: boolean
+  errors: Record<string, string>
+  sanitized: {
+    title: string
+    description: string | null
+    priority: 'low' | 'medium' | 'high'
+    dueDate: Date | null
+    tags: string[]
+    notes: string | null
+  }
+} {
+  const errors: Record<string, string> = {}
+  
+  // Validate title
+  const titleResult = validateItemTitle(data.title)
+  if (!titleResult.isValid) {
+    errors.title = titleResult.error!
+  }
+  
+  // Validate description
+  const descResult = validateDescription(data.description)
+  if (!descResult.isValid) {
+    errors.description = descResult.error!
+  }
+  
+  // Validate priority
+  const priority = data.priority || 'medium'
+  const priorityResult = validatePriority(priority)
+  if (!priorityResult.isValid) {
+    errors.priority = priorityResult.error!
+  }
+  
+  // Validate due date
+  let dueDate: Date | null = null
+  if (data.dueDate) {
+    const dateResult = validateDateString(data.dueDate)
+    if (!dateResult.isValid) {
+      errors.dueDate = dateResult.error!
+    } else {
+      dueDate = new Date(data.dueDate)
+    }
+  }
+  
+  // Validate tags
+  const tags = data.tags || []
+  const tagsResult = validateTags(tags)
+  if (!tagsResult.isValid) {
+    errors.tags = tagsResult.error!
+  }
+  
+  // Validate notes
+  const notesResult = validateNotes(data.notes)
+  if (!notesResult.isValid) {
+    errors.notes = notesResult.error!
+  }
+  
+  return {
+    isValid: Object.keys(errors).length === 0,
+    errors,
+    sanitized: {
+      title: sanitizeString(data.title),
+      description: data.description ? sanitizeString(data.description) : null,
+      priority: priority as 'low' | 'medium' | 'high',
+      dueDate,
+      tags: tags.map(sanitizeString),
+      notes: data.notes ? sanitizeString(data.notes) : null,
+    },
+  }
+}