diff --git a/requirements.txt b/requirements.txt
index 5d9efb5..f2821b2 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 fastapi
 uvicorn
 httpx
-watchfiles
\ No newline at end of file
+watchfiles
+pytest
\ No newline at end of file
diff --git a/src/app.py b/src/app.py
index 4ebb1d9..246ff58 100644
--- a/src/app.py
+++ b/src/app.py
@@ -33,11 +33,47 @@
         "max_participants": 20,
         "participants": ["emma@mergington.edu", "sophia@mergington.edu"]
     },
+    "Debate Team": {
+        "description": "Develop critical thinking and public speaking skills through competitive debate",
+        "schedule": "Mondays and Wednesdays, 4:00 PM - 5:30 PM",
+        "max_participants": 15,
+        "participants": ["alex@mergington.edu"]
+    },
+    "Robotics Club": {
+        "description": "Design, build, and program robots for competitions",
+        "schedule": "Tuesdays, Wednesdays, Thursdays, 3:30 PM - 5:00 PM",
+        "max_participants": 25,
+        "participants": ["james@mergington.edu", "sarah@mergington.edu"]
+    },
     "Gym Class": {
         "description": "Physical education and sports activities",
         "schedule": "Mondays, Wednesdays, Fridays, 2:00 PM - 3:00 PM",
         "max_participants": 30,
         "participants": ["john@mergington.edu", "olivia@mergington.edu"]
+    },
+    "Basketball Team": {
+        "description": "Competitive basketball training and games",
+        "schedule": "Tuesdays and Thursdays, 4:00 PM - 5:30 PM",
+        "max_participants": 20,
+        "participants": ["marcus@mergington.edu", "lucas@mergington.edu"]
+    },
+    "Track and Field": {
+        "description": "Training for running, jumping, and throwing events",
+        "schedule": "Mondays, Wednesdays, Fridays, 3:30 PM - 5:00 PM",
+        "max_participants": 25,
+        "participants": ["ava@mergington.edu"]
+    },
+    "Art Club": {
+        "description": "Explore painting, drawing, and sculpture with expert instructors",
+        "schedule": "Wednesdays, 3:30 PM - 5:00 PM",
+        "max_participants": 18,
+        "participants": ["isabella@mergington.edu", "grace@mergington.edu"]
+    },
+    "Music Ensemble": {
+        "description": "Join our orchestra, band, or choir and perform at school events",
+        "schedule": "Tuesdays and Fridays, 3:30 PM - 4:30 PM",
+        "max_participants": 40,
+        "participants": ["noah@mergington.edu", "mia@mergington.edu", "lucas@mergington.edu"]
     }
 }
 
@@ -62,6 +98,29 @@ def signup_for_activity(activity_name: str, email: str):
     # Get the specific activity
     activity = activities[activity_name]
 
+    # Validate student is not already signed up
+    if email in activity["participants"]:
+        raise HTTPException(status_code=400, detail="Student already signed up")
+
     # Add student
     activity["participants"].append(email)
     return {"message": f"Signed up {email} for {activity_name}"}
+
+
+@app.delete("/activities/{activity_name}/signup")
+def unregister_from_activity(activity_name: str, email: str):
+    """Unregister a student from an activity"""
+    # Validate activity exists
+    if activity_name not in activities:
+        raise HTTPException(status_code=404, detail="Activity not found")
+
+    # Get the specific activity
+    activity = activities[activity_name]
+
+    # Validate student is currently signed up
+    if email not in activity["participants"]:
+        raise HTTPException(status_code=404, detail="Student is not signed up for this activity")
+
+    # Remove student
+    activity["participants"].remove(email)
+    return {"message": f"Unregistered {email} from {activity_name}"}
diff --git a/src/static/app.js b/src/static/app.js
index dcc1e38..5f15bf1 100644
--- a/src/static/app.js
+++ b/src/static/app.js
@@ -4,14 +4,27 @@ document.addEventListener("DOMContentLoaded", () => {
   const signupForm = document.getElementById("signup-form");
   const messageDiv = document.getElementById("message");
 
+  function showMessage(text, type) {
+    messageDiv.textContent = text;
+    messageDiv.className = type;
+    messageDiv.classList.remove("hidden");
+
+    // Hide message after 5 seconds
+    setTimeout(() => {
+      messageDiv.classList.add("hidden");
+    }, 5000);
+  }
+
   // Function to fetch activities from API
   async function fetchActivities() {
     try {
-      const response = await fetch("/activities");
+      // Add a cache-busting query and disable cache to always show latest participant updates.
+      const response = await fetch(`/activities?ts=${Date.now()}`, { cache: "no-store" });
       const activities = await response.json();
 
       // Clear loading message
       activitiesList.innerHTML = "";
+      activitySelect.innerHTML = '<option value="">-- Select an activity --</option>';
 
       // Populate activities list
       Object.entries(activities).forEach(([name, details]) => {
@@ -19,12 +32,25 @@ document.addEventListener("DOMContentLoaded", () => {
         activityCard.className = "activity-card";
 
         const spotsLeft = details.max_participants - details.participants.length;
+        const participantsMarkup =
+          details.participants.length > 0
+            ? `<ul class="participants-list">${details.participants
+                .map(
+                  (participant) =>
+                    `<li><span class="participant-email">${participant}</span><button type="button" class="participant-delete" data-activity="${encodeURIComponent(name)}" data-email="${encodeURIComponent(participant)}" aria-label="Unregister ${participant}" title="Unregister participant">x</button></li>`
+                )
+                .join("")}</ul>`
+            : '<p class="participants-empty">No participants yet.</p>';
 
         activityCard.innerHTML = `
           <h4>${name}</h4>
           <p>${details.description}</p>
           <p><strong>Schedule:</strong> ${details.schedule}</p>
           <p><strong>Availability:</strong> ${spotsLeft} spots left</p>
+          <div class="participants-section">
+            <p class="participants-title"><strong>Participants</strong></p>
+            ${participantsMarkup}
+          </div>
         `;
 
         activitiesList.appendChild(activityCard);
@@ -53,31 +79,54 @@ document.addEventListener("DOMContentLoaded", () => {
         `/activities/${encodeURIComponent(activity)}/signup?email=${encodeURIComponent(email)}`,
         {
           method: "POST",
+          cache: "no-store",
         }
       );
 
       const result = await response.json();
 
       if (response.ok) {
-        messageDiv.textContent = result.message;
-        messageDiv.className = "success";
+        showMessage(result.message, "success");
         signupForm.reset();
+        await fetchActivities();
       } else {
-        messageDiv.textContent = result.detail || "An error occurred";
-        messageDiv.className = "error";
+        showMessage(result.detail || "An error occurred", "error");
       }
+    } catch (error) {
+      showMessage("Failed to sign up. Please try again.", "error");
+      console.error("Error signing up:", error);
+    }
+  });
+
+  activitiesList.addEventListener("click", async (event) => {
+    const deleteButton = event.target.closest(".participant-delete");
+    if (!deleteButton) {
+      return;
+    }
 
-      messageDiv.classList.remove("hidden");
+    const activityName = decodeURIComponent(deleteButton.dataset.activity || "");
+    const email = decodeURIComponent(deleteButton.dataset.email || "");
 
-      // Hide message after 5 seconds
-      setTimeout(() => {
-        messageDiv.classList.add("hidden");
-      }, 5000);
+    try {
+      const response = await fetch(
+        `/activities/${encodeURIComponent(activityName)}/signup?email=${encodeURIComponent(email)}`,
+        {
+          method: "DELETE",
+          cache: "no-store",
+        }
+      );
+
+      const result = await response.json();
+
+      if (response.ok) {
+        showMessage(result.message, "success");
+        await fetchActivities();
+      } else {
+        showMessage(result.detail || "Failed to unregister participant.", "error");
+      }
     } catch (error) {
-      messageDiv.textContent = "Failed to sign up. Please try again.";
-      messageDiv.className = "error";
-      messageDiv.classList.remove("hidden");
-      console.error("Error signing up:", error);
+      showMessage("Failed to unregister participant. Please try again.", "error");
+      console.error("Error unregistering participant:", error);
     }
   });
 
diff --git a/src/static/styles.css b/src/static/styles.css
index a533b32..fd39ce1 100644
--- a/src/static/styles.css
+++ b/src/static/styles.css
@@ -74,6 +74,67 @@ section h3 {
   margin-bottom: 8px;
 }
 
+.participants-section {
+  margin-top: 12px;
+  padding: 12px;
+  background: linear-gradient(135deg, #eef4ff 0%, #f7fbff 100%);
+  border: 1px solid #d8e6ff;
+  border-radius: 8px;
+}
+
+.participants-title {
+  margin-bottom: 8px;
+  color: #1a237e;
+  letter-spacing: 0.2px;
+}
+
+.participants-list {
+  margin: 0;
+  padding-left: 0;
+  list-style: none;
+}
+
+.participants-list li {
+  margin-bottom: 6px;
+  color: #29406b;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 10px;
+}
+
+.participants-list li:last-child {
+  margin-bottom: 0;
+}
+
+.participant-email {
+  overflow-wrap: anywhere;
+}
+
+.participant-delete {
+  width: 24px;
+  height: 24px;
+  border-radius: 50%;
+  border: 1px solid #d95757;
+  background-color: #fff;
+  color: #c62828;
+  font-weight: bold;
+  line-height: 1;
+  cursor: pointer;
+  padding: 0;
+  transition: background-color 0.2s ease, color 0.2s ease;
+}
+
+.participant-delete:hover {
+  background-color: #ffebee;
+}
+
+.participants-empty {
+  margin: 0;
+  color: #5f6b7a;
+  font-style: italic;
+}
+
 .form-group {
   margin-bottom: 15px;
 }
diff --git a/tests/test_app.py b/tests/test_app.py
new file mode 100644
index 0000000..9c73cc0
--- /dev/null
+++ b/tests/test_app.py
@@ -0,0 +1,122 @@
+"""Backend API tests for the FastAPI activity service."""
+
+from copy import deepcopy
+
+import pytest
+from fastapi.testclient import TestClient
+
+from src.app import activities, app
+
+client = TestClient(app)
+
+
+@pytest.fixture(autouse=True)
+def reset_activities_state():
+    """Reset in-memory activity data before each test for isolation."""
+    # Arrange
+    snapshot = deepcopy(activities)
+
+    # Act
+    yield
+
+    # Assert
+    activities.clear()
+    activities.update(snapshot)
+
+
+def test_get_activities_returns_all_activities():
+    # Arrange
+    expected_keys = {"description", "schedule", "max_participants", "participants"}
+
+    # Act
+    response = client.get("/activities")
+
+    # Assert
+    assert response.status_code == 200
+    data = response.json()
+    assert isinstance(data, dict)
+    assert data
+
+    for activity in data.values():
+        assert expected_keys.issubset(activity.keys())
+
+
+def test_signup_for_activity_succeeds_for_new_student():
+    # Arrange
+    activity_name = "Chess Club"
+    email = "new.student@mergington.edu"
+
+    # Act
+    response = client.post(f"/activities/{activity_name}/signup", params={"email": email})
+
+    # Assert
+    assert response.status_code == 200
+    assert response.json() == {"message": f"Signed up {email} for {activity_name}"}
+    assert email in activities[activity_name]["participants"]
+
+
+def test_signup_for_activity_fails_when_already_signed_up():
+    # Arrange
+    activity_name = "Chess Club"
+    email = "michael@mergington.edu"
+
+    # Act
+    response = client.post(f"/activities/{activity_name}/signup", params={"email": email})
+
+    # Assert
+    assert response.status_code == 400
+    assert response.json() == {"detail": "Student already signed up"}
+
+
+def test_signup_for_activity_fails_for_unknown_activity():
+    # Arrange
+    activity_name = "Unknown Club"
+    email = "student@mergington.edu"
+
+    # Act
+    response = client.post(f"/activities/{activity_name}/signup", params={"email": email})
+
+    # Assert
+    assert response.status_code == 404
+    assert response.json() == {"detail": "Activity not found"}
+
+
+def test_unregister_from_activity_succeeds_for_signed_up_student():
+    # Arrange
+    activity_name = "Chess Club"
+    email = "michael@mergington.edu"
+    assert email in activities[activity_name]["participants"]
+
+    # Act
+    response = client.delete(f"/activities/{activity_name}/signup", params={"email": email})
+
+    # Assert
+    assert response.status_code == 200
+    assert response.json() == {"message": f"Unregistered {email} from {activity_name}"}
+    assert email not in activities[activity_name]["participants"]
+
+
+def test_unregister_from_activity_fails_for_unknown_activity():
+    # Arrange
+    activity_name = "Unknown Club"
+    email = "student@mergington.edu"
+
+    # Act
+    response = client.delete(f"/activities/{activity_name}/signup", params={"email": email})
+
+    # Assert
+    assert response.status_code == 404
+    assert response.json() == {"detail": "Activity not found"}
+
+
+def test_unregister_from_activity_fails_when_student_not_signed_up():
+    # Arrange
+    activity_name = "Chess Club"
+    email = "not.signed.up@mergington.edu"
+
+    # Act
+    response = client.delete(f"/activities/{activity_name}/signup", params={"email": email})
+
+    # Assert
+    assert response.status_code == 404
+    assert response.json() == {"detail": "Student is not signed up for this activity"}