diff --git a/.kiro/specs/text-to-audiobook/tasks.md b/.kiro/specs/text-to-audiobook/tasks.md
index dfd32c7..6f1031a 100644
--- a/.kiro/specs/text-to-audiobook/tasks.md
+++ b/.kiro/specs/text-to-audiobook/tasks.md
@@ -152,7 +152,7 @@
   - Mock fetch calls and verify request format
   - _Requirements: 9.6_
 
-- [ ] 9. Implement text input component
+- [x] 9. Implement text input component
   - Create TextInputSection component
   - Add large multiline textarea
   - Add "Next: Assign voices" button
@@ -172,7 +172,7 @@
   - Test validation message appears for empty text
   - _Requirements: 1.1, 1.3, 1.4_
 
-- [ ] 10. Implement voice assignment component
+- [x] 10. Implement voice assignment component
   - Create VoiceAssignmentSection component
   - Display "Assign voices to roles" title
   - Render role list with labels and dropdowns
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index b36760d..52ba563 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -1,27 +1,102 @@
 import { useState } from 'react'
 import './App.css'
 import TextInputSection from './components/TextInputSection'
+import VoiceAssignmentSection from './components/VoiceAssignmentSection'
+import type { Role } from './api/tts'
+import { detectRoles } from './api/tts'
 
 function App() {
   const [text, setText] = useState('')
   const [phase, setPhase] = useState<'input' | 'assignment' | 'generated'>('input')
-  const [isLoading] = useState(false)
+  const [isLoading, setIsLoading] = useState(false)
+  const [roles, setRoles] = useState<Role[]>([])
+  const [voiceAssignments, setVoiceAssignments] = useState<Map<string, string>>(new Map())
+  const [language, setLanguage] = useState('a') // Default: American English
+  const [speed, setSpeed] = useState(1.0) // Default: 1.0x
+  const [isGenerating, setIsGenerating] = useState(false)
+  const [error, setError] = useState<string | null>(null)
 
   const handleTextChange = (newText: string) => {
     setText(newText)
   }
 
-  const handleNext = () => {
-    // TODO: Implement role detection and phase transition
-    console.log('Moving to voice assignment phase with text:', text)
-    setPhase('assignment')
+  const handleNext = async () => {
+    setIsLoading(true)
+    setError(null)
+    
+    try {
+      // Detect roles in the text
+      const response = await detectRoles(text)
+      setRoles(response.roles)
+      
+      // Initialize voice assignments map (empty initially)
+      setVoiceAssignments(new Map())
+      
+      // Move to assignment phase
+      setPhase('assignment')
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'Failed to detect roles')
+      console.error('Role detection error:', err)
+    } finally {
+      setIsLoading(false)
+    }
   }
 
+  const handleVoiceChange = (roleId: string, voiceId: string) => {
+    setVoiceAssignments(prev => {
+      const newMap = new Map(prev)
+      newMap.set(roleId, voiceId)
+      return newMap
+    })
+  }
+
+  const handleLanguageChange = (lang: string) => {
+    setLanguage(lang)
+  }
+
+  const handleSpeedChange = (newSpeed: number) => {
+    setSpeed(newSpeed)
+  }
+
+  const handleGenerate = async () => {
+    setIsGenerating(true)
+    setError(null)
+    
+    try {
+      // TODO: Implement audio generation
+      console.log('Generating audiobook with:', {
+        text,
+        language,
+        voiceAssignments: Array.from(voiceAssignments.entries()),
+        speed
+      })
+      
+      // Placeholder - will be implemented in later tasks
+      setTimeout(() => {
+        setIsGenerating(false)
+        setPhase('generated')
+      }, 1000)
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'Failed to generate audiobook')
+      console.error('Generation error:', err)
+      setIsGenerating(false)
+    }
+  }
+
+  // Check if all roles have assigned voices
+  const allRolesAssigned = roles.length > 0 && roles.every(role => voiceAssignments.has(role.id))
+
   return (
     <div className="min-h-screen bg-gray-50">
       <div className="container mx-auto py-8">
         <h1 className="text-4xl font-bold text-center mb-8">Text to Audiobook</h1>
         
+        {error && (
+          <div className="max-w-4xl mx-auto mb-4 p-4 bg-red-100 border border-red-400 text-red-700 rounded">
+            {error}
+          </div>
+        )}
+        
         {phase === 'input' && (
           <TextInputSection
             text={text}
@@ -33,9 +108,18 @@ function App() {
         )}
 
         {phase === 'assignment' && (
-          <div className="text-center">
-            <p>Voice assignment phase (to be implemented)</p>
-          </div>
+          <VoiceAssignmentSection
+            roles={roles}
+            voiceAssignments={voiceAssignments}
+            onVoiceChange={handleVoiceChange}
+            language={language}
+            onLanguageChange={handleLanguageChange}
+            speed={speed}
+            onSpeedChange={handleSpeedChange}
+            onGenerate={handleGenerate}
+            isGenerating={isGenerating}
+            allRolesAssigned={allRolesAssigned}
+          />
         )}
 
         {phase === 'generated' && (
diff --git a/frontend/src/components/VoiceAssignmentSection.tsx b/frontend/src/components/VoiceAssignmentSection.tsx
new file mode 100644
index 0000000..94ec9d4
--- /dev/null
+++ b/frontend/src/components/VoiceAssignmentSection.tsx
@@ -0,0 +1,165 @@
+import type { Role } from '../api/tts';
+
+interface VoiceAssignmentSectionProps {
+  roles: Role[];
+  voiceAssignments: Map<string, string>;
+  onVoiceChange: (roleId: string, voiceId: string) => void;
+  language: string;
+  onLanguageChange: (lang: string) => void;
+  speed: number;
+  onSpeedChange: (speed: number) => void;
+  onGenerate: () => void;
+  isGenerating: boolean;
+  allRolesAssigned: boolean;
+}
+
+interface VoiceOption {
+  id: string;
+  label: string;
+  gender: 'male' | 'female';
+  accent: string;
+}
+
+interface LanguageOption {
+  code: string;
+  label: string;
+}
+
+// Kokoro voice options as defined in the design document
+const KOKORO_VOICES: VoiceOption[] = [
+  { id: 'af_bella', label: 'Bella (Female, American)', gender: 'female', accent: 'American' },
+  { id: 'af_sarah', label: 'Sarah (Female, American)', gender: 'female', accent: 'American' },
+  { id: 'am_adam', label: 'Adam (Male, American)', gender: 'male', accent: 'American' },
+  { id: 'am_michael', label: 'Michael (Male, American)', gender: 'male', accent: 'American' },
+  { id: 'bf_emma', label: 'Emma (Female, British)', gender: 'female', accent: 'British' },
+  { id: 'bf_isabella', label: 'Isabella (Female, British)', gender: 'female', accent: 'British' },
+  { id: 'bm_george', label: 'George (Male, British)', gender: 'male', accent: 'British' },
+  { id: 'bm_lewis', label: 'Lewis (Male, British)', gender: 'male', accent: 'British' },
+];
+
+// Language options as defined in the design document
+const LANGUAGES: LanguageOption[] = [
+  { code: 'a', label: 'American English' },
+  { code: 'b', label: 'British English' },
+  { code: 'f', label: 'French' },
+  { code: 'j', label: 'Japanese' },
+];
+
+export default function VoiceAssignmentSection({
+  roles,
+  voiceAssignments,
+  onVoiceChange,
+  language,
+  onLanguageChange,
+  speed,
+  onSpeedChange,
+  onGenerate,
+  isGenerating,
+  allRolesAssigned,
+}: VoiceAssignmentSectionProps) {
+  const handleVoiceSelect = (roleId: string, voiceId: string) => {
+    onVoiceChange(roleId, voiceId);
+  };
+
+  const handleLanguageSelect = (e: React.ChangeEvent<HTMLSelectElement>) => {
+    onLanguageChange(e.target.value);
+  };
+
+  const handleSpeedChange = (e: React.ChangeEvent<HTMLInputElement>) => {
+    const newSpeed = parseFloat(e.target.value);
+    onSpeedChange(newSpeed);
+  };
+
+  const isGenerateDisabled = !allRolesAssigned || isGenerating;
+
+  return (
+    <div className="w-full max-w-4xl mx-auto p-6">
+      <h2 className="text-2xl font-bold mb-6">Assign voices to roles</h2>
+
+      {/* Role-to-Voice Mapping Section */}
+      <div className="mb-8">
+        <h3 className="text-xl font-semibold mb-4">Voice Assignments</h3>
+        <div className="space-y-4">
+          {roles.map((role) => (
+            <div key={role.id} className="flex items-center gap-4">
+              <label className="w-40 font-medium text-gray-700">
+                {role.label}:
+              </label>
+              <select
+                value={voiceAssignments.get(role.id) || ''}
+                onChange={(e) => handleVoiceSelect(role.id, e.target.value)}
+                className="flex-1 p-2 border-2 border-gray-300 rounded-lg focus:border-blue-500 focus:outline-none"
+              >
+                <option value="">Select a voice...</option>
+                {KOKORO_VOICES.map((voice) => (
+                  <option key={voice.id} value={voice.id}>
+                    {voice.label}
+                  </option>
+                ))}
+              </select>
+            </div>
+          ))}
+        </div>
+      </div>
+
+      {/* Language Selector */}
+      <div className="mb-6">
+        <h3 className="text-xl font-semibold mb-4">Language</h3>
+        <select
+          value={language}
+          onChange={handleLanguageSelect}
+          className="w-full p-2 border-2 border-gray-300 rounded-lg focus:border-blue-500 focus:outline-none"
+        >
+          {LANGUAGES.map((lang) => (
+            <option key={lang.code} value={lang.code}>
+              {lang.label}
+            </option>
+          ))}
+        </select>
+      </div>
+
+      {/* Speed Slider */}
+      <div className="mb-8">
+        <h3 className="text-xl font-semibold mb-4">Playback Speed</h3>
+        <div className="flex items-center gap-4">
+          <span className="text-sm text-gray-600 w-12">0.5x</span>
+          <input
+            type="range"
+            min="0.5"
+            max="1.5"
+            step="0.1"
+            value={speed}
+            onChange={handleSpeedChange}
+            className="flex-1"
+          />
+          <span className="text-sm text-gray-600 w-12">1.5x</span>
+          <span className="font-semibold text-gray-800 w-16 text-center">
+            {speed.toFixed(1)}x
+          </span>
+        </div>
+      </div>
+
+      {/* Generate Button */}
+      <div className="flex justify-center">
+        <button
+          onClick={onGenerate}
+          disabled={isGenerateDisabled}
+          className={`px-8 py-3 rounded-lg font-semibold text-white transition-colors ${
+            isGenerateDisabled
+              ? 'bg-gray-400 cursor-not-allowed'
+              : 'bg-green-600 hover:bg-green-700 cursor-pointer'
+          }`}
+        >
+          {isGenerating ? 'Generating...' : 'Generate Audiobook'}
+        </button>
+      </div>
+
+      {/* Validation Message */}
+      {!allRolesAssigned && !isGenerating && (
+        <div className="mt-4 p-3 bg-yellow-100 border border-yellow-400 text-yellow-800 rounded text-center">
+          Please assign voices to all roles before generating the audiobook.
+        </div>
+      )}
+    </div>
+  );
+}