diff --git a/Cargo.lock b/Cargo.lock
index ee4f5e6..7621658 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1319,6 +1319,7 @@ dependencies = [
  "bevy_egui",
  "noise",
  "paste",
+ "rand 0.8.5",
  "serde",
 ]
 
diff --git a/crates/border-wars/Cargo.toml b/crates/border-wars/Cargo.toml
index fabf283..68e7ebf 100644
--- a/crates/border-wars/Cargo.toml
+++ b/crates/border-wars/Cargo.toml
@@ -17,3 +17,4 @@ noise = "0.8.2"
 paste = "1.0.14"
 bevnet = { path = "../bevnet" }
 serde = "1.0.197"
+rand = "0.8.5"
diff --git a/crates/border-wars/src/map/generation.rs b/crates/border-wars/src/map/generation.rs
index f1ce9ee..4f9088e 100644
--- a/crates/border-wars/src/map/generation.rs
+++ b/crates/border-wars/src/map/generation.rs
@@ -2,6 +2,7 @@
 
 use bevy::prelude::*;
 use noise::{NoiseFn, Perlin};
+use serde::{Deserialize, Serialize};
 
 use super::hex::*;
 use super::{Tile, TilePosition};
@@ -25,7 +26,7 @@ impl Plugin for MapGenerationPlugin {
 }
 
 /// An event to trigger the generation of the map.
-#[derive(Event)]
+#[derive(Event, Serialize, Deserialize, Clone, Copy)]
 pub struct StartMapGeneration {
     /// The seed used to generate the map.
     pub seed: u32,
diff --git a/crates/border-wars/src/networking/mod.rs b/crates/border-wars/src/networking/mod.rs
index 197eab5..9b31654 100644
--- a/crates/border-wars/src/networking/mod.rs
+++ b/crates/border-wars/src/networking/mod.rs
@@ -1,11 +1,13 @@
 //! All the code related to the networking.
 
-use bevnet::NetworkPlugin;
+use bevnet::{NetworkAppExt, NetworkPlugin, Receive};
 use bevy::prelude::*;
 use serde::{Deserialize, Serialize};
 
 use self::check_connection::CheckConnectionPlugin;
 use self::connection::ConnectionPlugin;
+use crate::map::generation::StartMapGeneration;
+use crate::CurrentScene;
 
 pub mod check_connection;
 pub mod connection;
@@ -17,6 +19,8 @@ impl Plugin for NetworkingPlugin {
     fn build(&self, app: &mut App) {
         app.add_plugins(NetworkPlugin::new("relay.cocosol.fr".to_string()))
             .add_plugins(ConnectionPlugin)
+            .add_systems(Update, handle_start_game)
+            .add_network_event::<StartGame>()
             .add_plugins(CheckConnectionPlugin);
     }
 }
@@ -33,3 +37,19 @@ pub enum PlayerRank {
     /// The player. He can join the game and play.
     Player,
 }
+
+/// The event to start the game, that is send by the admin.
+#[derive(Event, Serialize, Deserialize)]
+pub struct StartGame(pub StartMapGeneration);
+
+/// A fonction that handle the start of the game.
+fn handle_start_game(
+    mut next_stats: ResMut<NextState<CurrentScene>>,
+    mut start_game_events: EventReader<Receive<StartGame>>,
+    mut start_map_generation_writer: EventWriter<StartMapGeneration>,
+) {
+    for event in start_game_events.read() {
+        next_stats.set(CurrentScene::Game);
+        start_map_generation_writer.send(event.1.0);
+    }
+}
diff --git a/crates/border-wars/src/scenes/lobby.rs b/crates/border-wars/src/scenes/lobby.rs
index b734f2c..d3762cc 100644
--- a/crates/border-wars/src/scenes/lobby.rs
+++ b/crates/border-wars/src/scenes/lobby.rs
@@ -3,9 +3,11 @@
 use bevnet::{Connection, SendTo};
 use bevy::prelude::*;
 use bevy_egui::{egui, EguiContexts};
+use rand::Rng;
 
+use crate::map::generation::StartMapGeneration;
 use crate::networking::connection::RemovePlayer;
-use crate::networking::PlayerRank;
+use crate::networking::{PlayerRank, StartGame};
 use crate::{CurrentScene, Player};
 
 /// The plugin for the lobby.
@@ -20,10 +22,11 @@ impl Plugin for LobbyPlugin {
 /// Display the UI of the lobby.
 fn lobby_ui(
     mut ctx: EguiContexts,
-    mut next_scene: ResMut<NextState<CurrentScene>>,
     connection: Res<Connection>,
     all_players_query: Query<&Player>,
     mut kick_player: EventWriter<SendTo<RemovePlayer>>,
+    mut map_size: Local<u32>,
+    mut start_game_event: EventWriter<SendTo<StartGame>>,
 ) {
     // Get our player info.
     let Some(self_player) = all_players_query
@@ -62,9 +65,44 @@ fn lobby_ui(
             ui.separator();
         }
 
-        if self_player.rank == PlayerRank::Admin && ui.button("Run the game").clicked() {
-            next_scene.set(CurrentScene::Game);
-            // TODO: run the game
+        if self_player.rank != PlayerRank::Admin {
+            return;
+        }
+
+        ui.add(egui::Slider::new(&mut (*map_size), 1..=5).text("map size"));
+
+        if !ui.button("Run the game").clicked() {
+            return;
+        }
+
+        let seed = rand::thread_rng().gen::<u32>();
+        let index = *map_size as usize;
+        let nomber_of_players = all_players_query.iter().count() as u32;
+
+        let radius = get_map_sizes(nomber_of_players)[index] as u16 * 2;
+
+        // Start the game.
+        for player in all_players_query.iter() {
+            start_game_event.send(SendTo(
+                player.uuid,
+                StartGame(StartMapGeneration { seed, radius }),
+            ));
         }
     });
 }
+
+/// Get the map sizes form a given number of players.
+fn get_map_sizes(number_of_players: u32) -> Vec<u32> {
+    let mut result = Vec::with_capacity(6);
+
+    let mut current = 0;
+    while result.len() < 6 {
+        current += 1;
+
+        if (current * 6) % number_of_players == 0 {
+            result.push(current);
+        }
+    }
+
+    result
+}