diff --git a/.DS_Store b/.DS_Store
index 48f3106..6356478 100644
Binary files a/.DS_Store and b/.DS_Store differ
diff --git a/alien.py b/alien.py
new file mode 100644
index 0000000..c92933c
--- /dev/null
+++ b/alien.py
@@ -0,0 +1,24 @@
+import pygame
+from pygame.sprite import Sprite
+
+class Alien(Sprite):
+    """A class to represent a single alien in the fleet."""
+    def __init__(self, ai_game):
+        super().__init__()
+        self.screen = ai_game.screen
+        self.settings = ai_game.settings
+        self.image = pygame.image.load("images/alien.bmp")
+        self.rect = self.image.get_rect()
+        self.rect.x = self.rect.width
+        self.rect.y = self.rect.height
+        self.x = float(self.rect.x)
+
+    def check_edges(self):
+        """Return True if alien is at edge of screen."""
+        screen_rect = self.screen.get_rect()
+        return self.rect.right >= screen_rect.right or self.rect.left <= 0
+
+    def update(self):
+        """Move the alien right or left."""
+        self.x += self.settings.alien_speed * self.settings.fleet_direction
+        self.rect.x = self.x
diff --git a/alien_invasion.py b/alien_invasion.py
index d658390..b4e9392 100644
--- a/alien_invasion.py
+++ b/alien_invasion.py
@@ -1,8 +1,11 @@
 import sys
+from time import sleep
 import pygame
 from settings import Settings
+from game_stats import GameStats
 from ship import Ship
 from bullet import Bullet
+from alien import Alien
 from pygame.sprite import Group
 
 
@@ -18,20 +21,59 @@ class AlienInvasion:
     self.settings.screen_height = self.screen.get_rect().height
     pygame.display.set_caption("Alien Invasion")
     self.bg_color = self.settings.bg_color
+    self.stats = GameStats(self)
     self.ship = Ship(self)
     self.bullets = Group()
+    self.aliens = Group()
+    self._create_fleet()
 
   def run_game(self):
     """Start the main loop for the game."""
     while True:
       # Check for events and update the game state.
       self._check_events()
-      self.ship.update()
-      self._update_bullets()
+      if self.stats.game_active:
+        # Update the ship, bullets, and aliens.
+        self.ship.update()
+        self._update_bullets()
+        self._update_aliens()
       # Redraw the screen during each pass through the loop.
       self._update_screen()
       self.clock.tick(60)
 
+  def _create_fleet(self):
+      """Create a fleet of aliens."""
+      alien = Alien(self)
+      alien_width, alien_height = alien.rect.size
+      current_x, current_y = alien_width, alien_height
+      while current_y < (self.settings.screen_height - 3 * alien_height):
+        while current_x < (self.settings.screen_width - 2 * alien_width):
+          self._create_alien(current_x, current_y)
+          current_x += 2 * alien_width
+        current_x = alien_width  
+        current_y += 2 * alien_height
+
+  def _create_alien(self, x_position, y_position):
+      """Create an alien and place it in the fleet."""
+      alien = Alien(self)
+      alien.x = x_position
+      alien.rect.x = x_position
+      alien.rect.y = y_position
+      self.aliens.add(alien)
+
+  def _check_fleet_edges(self):
+      """Respond appropriately if any aliens have reached an edge."""
+      for alien in self.aliens.sprites():
+          if alien.check_edges():
+              self._change_fleet_direction()
+              break
+
+  def _change_fleet_direction(self):
+      """Drop the entire fleet and change its direction."""
+      for alien in self.aliens.sprites():
+          alien.rect.y += self.settings.fleet_drop_speed
+      self.settings.fleet_direction *= -1
+
   def _check_events(self):
     """Respond to keypresses and mouse events."""
     for event in pygame.event.get():
@@ -73,6 +115,45 @@ class AlienInvasion:
     for bullet in self.bullets.copy():
       if bullet.rect.bottom <= 0:
         self.bullets.remove(bullet)
+    self._check_bullet_alien_collisions()
+
+  def _check_alien_bottom(self):
+    """Check if any aliens have reached the bottom of the screen."""
+    for alien in self.aliens.sprites():
+      if alien.rect.bottom >= self.settings.screen_height:
+        self._ship_hit()
+        break
+
+  def _check_bullet_alien_collisions(self):
+    """Respond to bullet-alien collisions."""
+    collisions = pygame.sprite.groupcollide(self.bullets, self.aliens, True, True)
+    if not self.aliens:
+      # If the fleet is empty, create a new fleet.
+      self.bullets.empty()
+      self._create_fleet()
+
+  def _ship_hit(self):
+    """Respond to the ship being hit by an alien."""
+    if self.stats.ships_left > 0:
+      # Decrement ships_left and reset the game state.
+      self.stats.ships_left -= 1
+      self.bullets.empty()
+      self.aliens.empty()
+      self._create_fleet()
+      self.ship.center_ship()
+      sleep(0.5)
+    else:
+      self.stats.game_active = False
+      pygame.mouse.set_visible(True)
+
+  def _update_aliens(self):
+    """Update the positions of aliens."""
+    self._check_fleet_edges()
+    self.aliens.update()
+    if pygame.sprite.spritecollideany(self.ship, self.aliens):
+      self._ship_hit()
+      self._check_bullet_alien_collisions()
+    self._check_alien_bottom()
 
   def _update_screen(self):
     """Update images on the screen and flip to the new screen."""
@@ -80,6 +161,7 @@ class AlienInvasion:
     for bullet in self.bullets.sprites():
       bullet.draw_bullet()
     self.ship.blitme()
+    self.aliens.draw(self.screen)
     pygame.display.flip()
 
 if __name__ == '__main__':
diff --git a/game_stats.py b/game_stats.py
new file mode 100644
index 0000000..2c361ac
--- /dev/null
+++ b/game_stats.py
@@ -0,0 +1,14 @@
+class GameStats:
+    """Track statistics for Alien Invasion."""
+
+    def __init__(self, ai_game):
+        """Initialize statistics."""
+        self.settings = ai_game.settings
+        self.reset_stats()
+        # Start Alien Invasion in an active state.
+        self.game_active = True
+
+    def reset_stats(self):
+        """Initialize statistics that can change during the game."""
+        self.ships_left = self.settings.ship_limit
+        self.score = 0
\ No newline at end of file
diff --git a/images/alien.bmp b/images/alien.bmp
new file mode 100644
index 0000000..dbe01a0
Binary files /dev/null and b/images/alien.bmp differ
diff --git a/settings.py b/settings.py
index c066102..93f7b7d 100644
--- a/settings.py
+++ b/settings.py
@@ -9,6 +9,7 @@ class Settings:
 
     # Ship settings
     self.ship_speed = 1.5
+    self.ship_limit = 3
 
     # Bullet settings
     self.bullet_speed = 3.0
diff --git a/ship.py b/ship.py
index 6a90b47..caa28be 100644
--- a/ship.py
+++ b/ship.py
@@ -26,3 +26,8 @@ class Ship:
   def blitme(self):
     """Draw the ship at its current location."""
     self.screen.blit(self.image, self.rect)
+
+  def center_ship(self):
+    """Center the ship on the screen."""
+    self.rect.midbottom = self.screen.get_rect().midbottom
+    self.x = float(self.rect.x)