diff --git a/backend/src/quizzes/entities/quiz.entity.ts b/backend/src/quizzes/entities/quiz.entity.ts
index cc8685d..a0c1ea6 100644
--- a/backend/src/quizzes/entities/quiz.entity.ts
+++ b/backend/src/quizzes/entities/quiz.entity.ts
@@ -32,12 +32,12 @@ export class Quiz {
   @Column()
   passingScore: number;
 
-  @Column()
-  timeLimit: number;
-
-  @Column()
+  @Column({ type: 'int', default: 1 })
   maxAttempts: number;
 
+  @Column({ type: 'int', nullable: true })
+  timeLimit: number;
+
   @OneToMany(() => QuizQuestion, (quizQuestion) => quizQuestion.quiz)
   questions: QuizQuestion[];
 
diff --git a/backend/src/quizzes/quizzes.service.ts b/backend/src/quizzes/quizzes.service.ts
index f221ccd..acb5f47 100644
--- a/backend/src/quizzes/quizzes.service.ts
+++ b/backend/src/quizzes/quizzes.service.ts
@@ -1,26 +1,129 @@
-import { Injectable } from '@nestjs/common';
+import {
+  Injectable,
+  ForbiddenException,
+  NotFoundException,
+} from '@nestjs/common';
+import { InjectRepository } from '@nestjs/typeorm';
+import { Repository } from 'typeorm';
+import { Quiz } from './entities/quiz.entity';
 import { CreateQuizDto } from './dto/create-quiz.dto';
 import { UpdateQuizDto } from './dto/update-quiz.dto';
+import { SubmitQuizAttemptDto } from './dto/submit-quiz-attempt.dto';
+import { QuizAttempt } from './entities/quiz-attempt.entity';
 
 @Injectable()
 export class QuizzesService {
+  constructor(
+    @InjectRepository(Quiz)
+    private readonly quizRepo: Repository<Quiz>,
+
+    @InjectRepository(QuizAttempt)
+    private readonly submissionRepo: Repository<QuizAttempt>,
+  ) {}
+
+  // Create a new quiz
   create(createQuizDto: CreateQuizDto) {
-    return 'This action adds a new quiz';
+    const quiz = this.quizRepo.create(createQuizDto);
+    return this.quizRepo.save(quiz);
   }
 
+  // Get all quizzes
   findAll() {
-    return `This action returns all quizzes`;
+    return this.quizRepo.find();
+  }
+
+  // Get a single quiz
+  async findOne(id: number) {
+    const quiz = await this.quizRepo.findOne({ where: { id } });
+    if (!quiz) throw new NotFoundException('Quiz not found');
+    return quiz;
+  }
+
+  // Update a quiz
+  async update(id: number, updateQuizDto: UpdateQuizDto) {
+    await this.quizRepo.update(id, updateQuizDto);
+    return this.findOne(id);
+  }
+
+  // Delete a quiz
+  async remove(id: number) {
+    await this.quizRepo.delete(id);
+    return { message: 'Quiz deleted' };
   }
 
-  findOne(id: number) {
-    return `This action returns a #${id} quiz`;
+  // Validate user's quiz submission
+  async validateQuizSubmission(
+    userId: string,
+    quizId: string | number,
+    submissionId?: string,
+  ) {
+    const quiz = await this.quizRepo.findOne({ where: { id: Number(quizId) } });
+
+    if (!quiz) throw new NotFoundException('Quiz not found');
+
+    const attempts = await this.submissionRepo.count({
+      where: { userId: String(userId), quizId: String(quizId) },
+    });
+    if (attempts >= quiz.maxAttempts) {
+      throw new ForbiddenException('Max attempts exceeded');
+    }
+
+    if (submissionId) {
+      const submission = await this.submissionRepo.findOne({
+        where: { id: submissionId },
+      });
+      if (!submission) throw new NotFoundException('Submission not found');
+
+      const elapsed =
+        (new Date().getTime() - new Date(submission.startTime).getTime()) /
+        1000;
+      if (quiz.timeLimit && elapsed > quiz.timeLimit) {
+        throw new ForbiddenException('Time limit exceeded');
+      }
+    }
   }
 
-  update(id: number, updateQuizDto: UpdateQuizDto) {
-    return `This action updates a #${id} quiz`;
+  // Handle quiz submission
+  async submitQuizAttempt(submissionId: string, dto: SubmitQuizAttemptDto) {
+    const submission = await this.submissionRepo.findOne({
+      where: { id: submissionId },
+      relations: ['quiz'],
+    });
+
+    if (!submission) throw new NotFoundException('Submission not found');
+
+    const now = new Date();
+    const elapsed =
+      (now.getTime() - new Date(submission.startTime).getTime()) / 1000;
+
+    if (submission.quiz.timeLimit && elapsed > submission.quiz.timeLimit) {
+      throw new ForbiddenException('Time limit exceeded');
+    }
+
+    submission.answers = dto.answers;
+    submission.endTime = now;
+
+    return this.submissionRepo.save(submission);
   }
 
-  remove(id: number) {
-    return `This action removes a #${id} quiz`;
+  async startQuizAttempt(userId: string, quizId: string) {
+    const quiz = await this.quizRepo.findOne({ where: { id: Number(quizId) } });
+    if (!quiz) throw new NotFoundException('Quiz not found');
+
+    const attempts = await this.submissionRepo.count({
+      where: { userId, quizId },
+    });
+    if (attempts >= quiz.maxAttempts) {
+      throw new ForbiddenException('Max attempts exceeded');
+    }
+
+    const submission = this.submissionRepo.create({
+      userId,
+      quizId,
+      attemptNumber: attempts + 1,
+      startTime: new Date(),
+    });
+
+    return this.submissionRepo.save(submission);
   }
 }