diff --git a/backend/.gitignore b/backend/.gitignore
index 239f1c2..f2b0a9f 100644
--- a/backend/.gitignore
+++ b/backend/.gitignore
@@ -32,4 +32,5 @@ build/
 ### VS Code ###
 .vscode/
 .env
-env.properties
\ No newline at end of file
+env.properties
+firebase-service-account.json
\ No newline at end of file
diff --git a/backend/pom.xml b/backend/pom.xml
index c085247..1af2abd 100644
--- a/backend/pom.xml
+++ b/backend/pom.xml
@@ -90,6 +90,11 @@
 			<artifactId>springdoc-openapi-starter-webmvc-ui</artifactId>
 			<version>2.3.0</version>
 		</dependency>
+		<dependency>
+			<groupId>com.google.firebase</groupId>
+			<artifactId>firebase-admin</artifactId>
+			<version>9.2.0</version>
+		</dependency>
 	</dependencies>
 
 	<build>
diff --git a/backend/src/main/java/online/wesal/wesal/WesalApplication.java b/backend/src/main/java/online/wesal/wesal/WesalApplication.java
index 5b2a4b8..f25819c 100644
--- a/backend/src/main/java/online/wesal/wesal/WesalApplication.java
+++ b/backend/src/main/java/online/wesal/wesal/WesalApplication.java
@@ -2,8 +2,10 @@ package online.wesal.wesal;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.scheduling.annotation.EnableAsync;
 
 @SpringBootApplication
+@EnableAsync
 public class WesalApplication {
 
 	public static void main(String[] args) {
diff --git a/backend/src/main/java/online/wesal/wesal/config/FirebaseConfig.java b/backend/src/main/java/online/wesal/wesal/config/FirebaseConfig.java
new file mode 100644
index 0000000..48ca0b3
--- /dev/null
+++ b/backend/src/main/java/online/wesal/wesal/config/FirebaseConfig.java
@@ -0,0 +1,29 @@
+package online.wesal.wesal.config;
+
+import com.google.auth.oauth2.GoogleCredentials;
+import com.google.firebase.FirebaseApp;
+import com.google.firebase.FirebaseOptions;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.io.ClassPathResource;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+@Configuration
+public class FirebaseConfig {
+
+    @Bean
+    public FirebaseApp firebaseApp() throws IOException {
+        if (FirebaseApp.getApps().isEmpty()) {
+            InputStream serviceAccount = new ClassPathResource("firebase-service-account.json").getInputStream();
+            
+            FirebaseOptions options = FirebaseOptions.builder()
+                    .setCredentials(GoogleCredentials.fromStream(serviceAccount))
+                    .build();
+
+            return FirebaseApp.initializeApp(options);
+        }
+        return FirebaseApp.getInstance();
+    }
+}
\ No newline at end of file
diff --git a/backend/src/main/java/online/wesal/wesal/service/FCMNotificationService.java b/backend/src/main/java/online/wesal/wesal/service/FCMNotificationService.java
new file mode 100644
index 0000000..9fd9f90
--- /dev/null
+++ b/backend/src/main/java/online/wesal/wesal/service/FCMNotificationService.java
@@ -0,0 +1,105 @@
+package online.wesal.wesal.service;
+
+import com.google.firebase.messaging.FirebaseMessaging;
+import com.google.firebase.messaging.Message;
+import com.google.firebase.messaging.Notification;
+import online.wesal.wesal.entity.User;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.scheduling.annotation.Async;
+import org.springframework.stereotype.Service;
+
+import java.util.List;
+
+@Service
+public class FCMNotificationService {
+
+    private static final Logger logger = LoggerFactory.getLogger(FCMNotificationService.class);
+
+    @Async
+    public void sendNotificationToUser(User user, String title, String body) {
+        if (user.getFcmToken() == null || user.getFcmToken().trim().isEmpty()) {
+            logger.warn("User {} has no FCM token, skipping notification", user.getDisplayName());
+            return;
+        }
+
+        try {
+            Message message = Message.builder()
+                    .setToken(user.getFcmToken())
+                    .setNotification(Notification.builder()
+                            .setTitle(title)
+                            .setBody(body)
+                            .build())
+                    .build();
+
+
+            String response = FirebaseMessaging.getInstance().send(message);
+            logger.info("Successfully sent notification to {}: {}", user.getDisplayName(), response);
+        } catch (Exception e) {
+            logger.error("Failed to send notification to {}: {}", user.getDisplayName(), e.getMessage());
+        }
+    }
+
+    @Async
+    public void sendNotificationToUsers(List<User> users, String title, String body) {
+        users.forEach(user -> sendNotificationToUser(user, title, body));
+    }
+
+    @Async
+    public void sendInvitationAcceptedNotification(String organizerToken, String organizerName, String accepterName, String tagName) {
+        String title = "New Attendee! 🎉";
+        String body = String.format("%s accepted your invite for %s", accepterName, tagName);
+        
+        if (organizerToken == null || organizerToken.trim().isEmpty()) {
+            logger.warn("User {} has no FCM token, skipping notification", organizerName);
+            return;
+        }
+
+        try {
+            Message message = Message.builder()
+                    .setToken(organizerToken)
+                    .setNotification(Notification.builder()
+                            .setTitle(title)
+                            .setBody(body)
+                            .build())
+                    .build();
+
+            String response = FirebaseMessaging.getInstance().send(message);
+            logger.info("Successfully sent notification to {}: {}", organizerName, response);
+        } catch (Exception e) {
+            logger.error("Failed to send notification to {}: {}", organizerName, e.getMessage());
+        }
+    }
+
+    @Async
+    public void sendInvitationFullNotification(List<String> fcmTokens, List<String> displayNames, String tagName, int totalAttendees) {
+        String title = String.format("You're set for %s! 🚀", tagName);
+        String body = String.format("%d people have accepted the invitation. Click to see who they are!", totalAttendees);
+        
+        for (int i = 0; i < fcmTokens.size(); i++) {
+            String token = fcmTokens.get(i);
+            String name = displayNames.get(i);
+            
+            if (token == null || token.trim().isEmpty()) {
+                logger.warn("User {} has no FCM token, skipping notification", name);
+                continue;
+            }
+
+            try {
+                Message message = Message.builder()
+                        .setToken(token)
+                        .setNotification(Notification.builder()
+                                .setTitle(title)
+                                .setBody(body)
+                                .build())
+                        .build();
+
+                String response = FirebaseMessaging.getInstance().send(message);
+                logger.info("Successfully sent notification to {}: {}", name, response);
+            } catch (Exception e) {
+                logger.error("Failed to send notification to {}: {}", name, e.getMessage());
+            }
+        }
+    }
+
+}
\ No newline at end of file
diff --git a/backend/src/main/java/online/wesal/wesal/service/InvitationService.java b/backend/src/main/java/online/wesal/wesal/service/InvitationService.java
index 57fbbdb..ac9638c 100644
--- a/backend/src/main/java/online/wesal/wesal/service/InvitationService.java
+++ b/backend/src/main/java/online/wesal/wesal/service/InvitationService.java
@@ -35,6 +35,9 @@ public class InvitationService {
     @Autowired
     private AttendeeRepository attendeeRepository;
     
+    @Autowired
+    private FCMNotificationService fcmNotificationService;
+    
     @Transactional
     public InvitationResponse createInvitation(CreateInvitationRequest request, String userEmail) {
         User creator = userRepository.findByEmail(userEmail)
@@ -118,7 +121,41 @@ public class InvitationService {
         attendeeRepository.save(attendee);
         
         invitation.setCurrentAttendees(invitation.getCurrentAttendees() + 1);
+        int newAttendeeCount = invitation.getCurrentAttendees();
         invitationRepository.save(invitation);
+        
+        // If invitation is now full, notify everyone
+        if (newAttendeeCount == invitation.getMaxParticipants()) {
+            // Get all participants (attendees + organizer)
+            List<Attendee> attendees = attendeeRepository.findByInvitationId(invitationId);
+            List<User> allParticipants = attendees.stream()
+                .map(Attendee::getUser)
+                .collect(Collectors.toList());
+            allParticipants.add(invitation.getCreator());
+            
+            // Extract FCM tokens and display names for async processing
+            List<String> fcmTokens = allParticipants.stream()
+                .map(User::getFcmToken)
+                .collect(Collectors.toList());
+            List<String> displayNames = allParticipants.stream()
+                .map(User::getDisplayName)
+                .collect(Collectors.toList());
+            
+            fcmNotificationService.sendInvitationFullNotification(
+                fcmTokens,
+                displayNames,
+                invitation.getTag().getName(),
+                newAttendeeCount
+            );
+        } else {
+            // Send notification to organizer about new attendee (only if not full)
+            fcmNotificationService.sendInvitationAcceptedNotification(
+                invitation.getCreator().getFcmToken(),
+                invitation.getCreator().getDisplayName(),
+                user.getDisplayName(),
+                invitation.getTag().getName()
+            );
+        }
     }
     
     @Transactional