diff --git a/src/app.js b/src/app.js
index 8fe3c6e..e1ff4e4 100644
--- a/src/app.js
+++ b/src/app.js
@@ -1,5 +1,5 @@
 import { LessonEngine } from "./impl/LessonEngine.js";
-import { renderLesson, renderModuleList, renderLevelIndicator, showFeedback } from "./helpers/renderer.js";
+import { renderLesson, renderModuleList, renderLevelIndicator, showFeedback, updateActiveLessonInSidebar } from "./helpers/renderer.js";
 import { validateUserCode } from "./helpers/validator.js";
 import { loadModules } from "./config/lessons.js";
 
@@ -89,7 +89,9 @@ function initFeedbackToggle() {
 async function initializeModules() {
 	try {
 		state.modules = await loadModules();
-		renderModuleList(elements.moduleList, state.modules, selectModule);
+
+		// Use the new renderModuleList function with both callbacks
+		renderModuleList(elements.moduleList, state.modules, selectModule, selectLesson);
 
 		// Select the first module or the last one user was on
 		const lastModuleId = localStorage.getItem("codeCrispies.lastModuleId");
@@ -158,8 +160,8 @@ function selectModule(moduleId) {
 
 	state.currentModule = selectedModule;
 
-	// Update module list UI
-	const moduleItems = elements.moduleList.querySelectorAll(".module-list-item");
+	// Update module list UI to highlight the active module
+	const moduleItems = elements.moduleList.querySelectorAll(".module-header");
 	moduleItems.forEach((item) => {
 		item.classList.remove("active");
 		if (item.dataset.moduleId === moduleId) {
@@ -182,6 +184,23 @@ function selectModule(moduleId) {
 	resetSuccessIndicators();
 }
 
+function selectLesson(moduleId, lessonIndex) {
+	// Select the module first if it's not already selected
+	if (!state.currentModule || state.currentModule.id !== moduleId) {
+		selectModule(moduleId);
+	}
+
+	// Update current lesson index
+	state.currentLessonIndex = lessonIndex;
+
+	// Update user progress
+	state.userProgress[moduleId].current = lessonIndex;
+	saveUserProgress();
+
+	// Load the lesson
+	loadCurrentLesson();
+}
+
 // Reset success indicators
 function resetSuccessIndicators() {
 	elements.codeEditor.classList.remove("success-highlight");
@@ -255,6 +274,9 @@ function loadCurrentLesson() {
 	// Update level indicator
 	renderLevelIndicator(elements.levelIndicator, state.currentLessonIndex + 1, state.currentModule.lessons.length);
 
+	// Update active lesson in sidebar
+	updateActiveLessonInSidebar(state.currentModule.id, state.currentLessonIndex);
+
 	// Update navigation buttons
 	updateNavigationButtons();
 
diff --git a/src/helpers/renderer.js b/src/helpers/renderer.js
index 707a370..c24ff08 100644
--- a/src/helpers/renderer.js
+++ b/src/helpers/renderer.js
@@ -7,40 +7,110 @@ let feedbackElement = null;
 let feedbackTimeout = null;
 
 /**
- * Render the module list in the sidebar
+ * Render the module list in the sidebar with expandable lessons
  * @param {HTMLElement} container - The container element for the module list
  * @param {Array} modules - The list of modules
  * @param {Function} onSelectModule - Callback when a module is selected
+ * @param {Function} onSelectLesson - Callback when a lesson is selected
  */
-export function renderModuleList(container, modules, onSelectModule) {
+export function renderModuleList(container, modules, onSelectModule, onSelectLesson) {
 	// Clear the container
 	container.innerHTML = "<h3>CSS Lessons</h3>";
 
+	// Get user progress from localStorage
+	const progressData = localStorage.getItem("codeCrispies.Progress");
+	let progress = {};
+	if (progressData) {
+		try {
+			progress = JSON.parse(progressData);
+		} catch (e) {
+			console.error("Error parsing progress data:", e);
+		}
+	}
+
 	// Create list items for each module
 	modules.forEach((module) => {
-		const moduleItem = document.createElement("div");
-		moduleItem.classList.add("module-list-item");
-		moduleItem.dataset.moduleId = module.id;
-		moduleItem.textContent = module.title;
+		// Create module container
+		const moduleContainer = document.createElement("div");
+		moduleContainer.classList.add("module-container");
 
-		// Get user progress from localStorage to mark completed lessons
-		const progressData = localStorage.getItem("codeCrispies.Progress");
-		if (progressData) {
-			try {
-				const progress = JSON.parse(progressData);
-				if (progress[module.id] && progress[module.id].completed.length === module.lessons.length) {
-					moduleItem.classList.add("completed");
-				}
-			} catch (e) {
-				console.error("Error parsing progress data:", e);
-			}
+		// Create module header item (clickable to expand/collapse)
+		const moduleHeader = document.createElement("div");
+		moduleHeader.classList.add("module-list-item", "module-header");
+		moduleHeader.dataset.moduleId = module.id;
+
+		// Create module title with expand/collapse indicator
+		const moduleTitle = document.createElement("span");
+		moduleTitle.classList.add("module-title");
+		moduleTitle.textContent = module.title;
+
+		// Create expand/collapse icon
+		const expandIcon = document.createElement("span");
+		expandIcon.classList.add("expand-icon");
+		expandIcon.innerHTML = "▶"; // Right-pointing triangle
+
+		moduleHeader.appendChild(expandIcon);
+		moduleHeader.appendChild(moduleTitle);
+
+		// Check if the module is completed
+		if (progress[module.id] && progress[module.id].completed.length === module.lessons.length) {
+			moduleHeader.classList.add("completed");
 		}
 
-		moduleItem.addEventListener("click", () => {
-			onSelectModule(module.id);
+		// Lessons container (initially hidden)
+		const lessonsContainer = document.createElement("div");
+		lessonsContainer.classList.add("lessons-container");
+		lessonsContainer.style.display = "none"; // Initially collapsed
+
+		// Create list items for each lesson in this module
+		module.lessons.forEach((lesson, index) => {
+			const lessonItem = document.createElement("div");
+			lessonItem.classList.add("lesson-list-item");
+			lessonItem.dataset.moduleId = module.id;
+			lessonItem.dataset.lessonIndex = index;
+			lessonItem.textContent = lesson.title || `Lesson ${index + 1}`;
+
+			// Mark lesson as completed if in progress data
+			if (progress[module.id] && progress[module.id].completed.includes(index)) {
+				lessonItem.classList.add("completed");
+			}
+
+			// Mark lesson as current if it's the current lesson
+			if (progress[module.id] && progress[module.id].current === index) {
+				lessonItem.classList.add("current");
+			}
+
+			// Add click event to select lesson
+			lessonItem.addEventListener("click", () => {
+				// Update UI to show this lesson is selected
+				document.querySelectorAll(".lesson-list-item").forEach((item) => {
+					item.classList.remove("active");
+				});
+				lessonItem.classList.add("active");
+
+				// Call the onSelectLesson callback
+				onSelectLesson(module.id, index);
+			});
+
+			lessonsContainer.appendChild(lessonItem);
 		});
 
-		container.appendChild(moduleItem);
+		// Toggle expand/collapse when clicking on module header
+		moduleHeader.addEventListener("click", () => {
+			// Toggle visibility of lessons container
+			const isExpanded = lessonsContainer.style.display !== "none";
+			lessonsContainer.style.display = isExpanded ? "none" : "block";
+
+			// Update expand/collapse icon
+			expandIcon.innerHTML = isExpanded ? "▶" : "▼";
+		});
+
+		// Add module header and lessons container to module container
+		moduleContainer.appendChild(moduleHeader);
+		moduleContainer.appendChild(lessonsContainer);
+
+		// Add the complete module container to the sidebar
+		container.appendChild(moduleContainer);
 	});
 }
 
@@ -132,3 +202,41 @@ export function clearFeedback() {
 	}
 	feedbackElement = null;
 }
+
+/**
+ * Update the active lesson in the sidebar
+ * @param {string} moduleId - The ID of the module
+ * @param {number} lessonIndex - The index of the lesson
+ */
+export function updateActiveLessonInSidebar(moduleId, lessonIndex) {
+	// Remove active class from all lessons
+	document.querySelectorAll(".lesson-list-item").forEach((item) => {
+		item.classList.remove("active");
+	});
+
+	// Find and activate the current lesson
+	const selector = `.lesson-list-item[data-module-id="${moduleId}"][data-lesson-index="${lessonIndex}"]`;
+	const currentLessonItem = document.querySelector(selector);
+
+	if (currentLessonItem) {
+		currentLessonItem.classList.add("active");
+
+		// Make sure parent module is expanded
+		const parentLessonsContainer = currentLessonItem.parentElement;
+		if (parentLessonsContainer && parentLessonsContainer.classList.contains("lessons-container")) {
+			parentLessonsContainer.style.display = "block";
+
+			// Update expand icon
+			const moduleHeader = parentLessonsContainer.previousElementSibling;
+			if (moduleHeader) {
+				const expandIcon = moduleHeader.querySelector(".expand-icon");
+				if (expandIcon) {
+					expandIcon.innerHTML = "▼"; // Down arrow when expanded
+				}
+			}
+
+			// Scroll to ensure the item is visible
+			currentLessonItem.scrollIntoView({ behavior: "smooth", block: "nearest" });
+		}
+	}
+}
diff --git a/src/main.css b/src/main.css
index 53791f4..36f65e3 100644
--- a/src/main.css
+++ b/src/main.css
@@ -794,6 +794,93 @@ input:checked + .toggle-slider:before {
 	}
 }
 
+/* Module and Lesson List Styles */
+.module-container {
+	margin-bottom: 8px;
+}
+
+.module-header {
+	display: flex;
+	align-items: center;
+	cursor: pointer;
+	padding: 8px 12px;
+	border-radius: 4px;
+	transition: background-color 0.2s;
+	font-weight: 600;
+}
+
+.module-header:hover {
+	background-color: var(--hover-color);
+}
+
+.expand-icon {
+	display: inline-block;
+	margin-right: 8px;
+	font-size: 10px;
+	transition: transform 0.2s;
+}
+
+.lessons-container {
+	margin-left: 16px;
+	border-left: 2px solid var(--border-color);
+	padding-left: 8px;
+}
+
+.lesson-list-item {
+	padding: 6px 12px;
+	border-radius: 4px;
+	cursor: pointer;
+	transition: background-color 0.2s;
+	font-size: 0.9em;
+	margin: 4px 0;
+}
+
+.lesson-list-item:hover {
+	background-color: var(--primary-bg-medium);
+}
+
+.lesson-list-item.active {
+	background-color: var(--primary-bg-medium);
+	color: var(--dark-text);
+	font-weight: bold;
+}
+
+.lesson-list-item.completed::before {
+	content: "✓";
+	margin-right: 6px;
+	color: var(--success-color);
+}
+
+.module-header.completed::before {
+	content: "✓";
+	margin-right: 6px;
+	color: var(--success-color);
+}
+
+/* Improve scrolling for the sidebar */
+.sidebar .module-list {
+	max-height: calc(100vh - 200px);
+	padding-right: 5px;
+}
+
+/* Add smooth scrolling */
+.sidebar {
+	scroll-behavior: smooth;
+}
+
+/* Mobile adjustments */
+@media (max-width: 768px) {
+	.lessons-container {
+		margin-left: 8px;
+		padding-left: 6px;
+	}
+
+	.lesson-list-item,
+	.module-header {
+		padding: 8px 6px;
+	}
+}
+
 /* ================= RESPONSIVE DESIGN ================= */
 
 /* Base responsive layout */