diff --git a/libbettola/include/bettola/game/player_base.h b/libbettola/include/bettola/game/player_base.h
index 07425d7..0944f80 100644
--- a/libbettola/include/bettola/game/player_base.h
+++ b/libbettola/include/bettola/game/player_base.h
@@ -15,7 +15,7 @@ public:
 
   PlayerBase(void);
 
-  void update(double dt);
+  void apply_gravity_and_collision(float terrain_height);
   void set_velocity_direction(const InputState& input, const BettolaMath::Vec3& cam_front);
   void set_position(const BettolaMath::Vec3& pos);
   void set_yaw(float yaw) { _yaw = yaw; }
@@ -24,6 +24,8 @@ public:
   const BettolaMath::Vec3& get_position(void) const { return _position; }
   float get_yaw(void)       const { return _yaw; }
 
+  void update(double dt);
+
 protected:
   unsigned int _id;
   BettolaMath::Vec3 _position;
diff --git a/libbettola/include/bettola/game/terrain.h b/libbettola/include/bettola/game/terrain.h
new file mode 100644
index 0000000..eeb2e55
--- /dev/null
+++ b/libbettola/include/bettola/game/terrain.h
@@ -0,0 +1,49 @@
+#pragma once
+
+#include <cmath>
+#include <algorithm>
+#include "bettola/game/chunk.h"
+
+namespace BettolaLib {
+namespace Game {
+namespace Terrain {
+
+/*
+ * Performs bilinear interpolation on a chunk's heightmap to find
+ * a smooth height at a given local coord.
+ */
+inline float get_height_at(const BettolaLib::Game::Chunk& chunk, float local_x, float local_z) {
+  /*
+   * Mesh generation uses (WIDTH-1), so coordinates for interpolation
+   * should be within the range [0, WIDTH-2].
+   */
+  float max_coord = CHUNK_WIDTH-2;
+  if(local_x < 0 || local_x > max_coord || local_z < 0 || local_z > max_coord) {
+    int x = std::max(0, std::min((int)local_x, CHUNK_WIDTH  - 1));
+    int z = std::max(0, std::min((int)local_z, CHUNK_HEIGHT - 1));
+    return chunk.heightmap[z*CHUNK_WIDTH+x] * 5.0f; /* Apply height scaling. */
+  }
+
+  /* Get integer and fracctional parts of the coords. */
+  int x_int = (int)local_x;
+  int z_int = (int)local_z;
+  float x_frac = local_x - x_int;
+  float z_frac = local_z - z_int;
+
+  /* Get heights of the four corners of the grid cell. */
+  float h00 = chunk.heightmap[z_int     * CHUNK_WIDTH+x_int];
+  float h10 = chunk.heightmap[z_int     * CHUNK_WIDTH+(x_int+1)];
+  float h01 = chunk.heightmap[(z_int+1) * CHUNK_WIDTH+x_int];
+  float h11 = chunk.heightmap[(z_int+1) * CHUNK_WIDTH+(x_int+1)];
+
+  /* Bilinear interpolation. */
+  float h_x0 = h00    * (1 - x_frac) + h10  * x_frac;
+  float h_x1 = h01    * (1 - x_frac) + h11  * x_frac;
+  float height = h_x0 * (1 - z_frac) + h_x1 * z_frac;
+
+  return height * 5.0f; /* Apply the same scaling used in ChunkMesh. */
+}
+
+} /* namespace Terrain. */
+} /* namespace Game. */
+} /* namespace BettolaLib. */
diff --git a/libbettola/include/bettola/network/player_state_message.h b/libbettola/include/bettola/network/player_state_message.h
index e94d602..016c469 100644
--- a/libbettola/include/bettola/network/player_state_message.h
+++ b/libbettola/include/bettola/network/player_state_message.h
@@ -9,6 +9,7 @@ struct PlayerStateMessage {
   unsigned int player_id;
   float x;
   float y;
+  float z;
   float yaw;
   unsigned int last_processed_sequence;
 };
diff --git a/libbettola/src/bettola/game/player_base.cpp b/libbettola/src/bettola/game/player_base.cpp
index 43f061b..23564e7 100644
--- a/libbettola/src/bettola/game/player_base.cpp
+++ b/libbettola/src/bettola/game/player_base.cpp
@@ -1,6 +1,7 @@
 #include <cmath>
 #include "bettola/game/player_base.h"
 #include "bettola/network/player_input_message.h"
+#include "game/player.h"
 
 /* Use a static variable for ID generation across all player types. */
 static unsigned int next_player_id = 1;
@@ -13,11 +14,21 @@ PlayerBase::PlayerBase(void) :
     _speed(20.0f) {}
 
 void PlayerBase::update(double dt) {
+  /* Apply gravity force. */
+  _velocity.y -= 30.0f * dt;
+
   _position.x += _velocity.x * dt;
   _position.y += _velocity.y * dt;
   _position.z += _velocity.z * dt;
 }
 
+void PlayerBase::apply_gravity_and_collision(float terrain_height) {
+  if(_position.y < terrain_height) {
+    _position.y = terrain_height;
+    _velocity.y = 0;
+  }
+}
+
 void PlayerBase::set_position(const BettolaMath::Vec3& pos) {
   _position = pos;
 }
@@ -40,11 +51,9 @@ void PlayerBase::set_velocity_direction(const InputState& input, const BettolaMa
   float move_mag = sqrt(move_dir.x*move_dir.x + move_dir.z*move_dir.z);
   if(move_mag > 0.0f) {
     _velocity.x = (move_dir.x / move_mag) * _speed;
-    _velocity.y = 0.0;
-    _velocity.z = (move_dir.z / move_mag) * _speed; /* y velocity is for z-axis. */
+    _velocity.z = (move_dir.z / move_mag) * _speed;
   } else {
     _velocity.x = 0.0f;
-    _velocity.y = 0.0f;
     _velocity.z = 0.0f;
   }
 }
diff --git a/src/bettola.cpp b/src/bettola.cpp
index f141241..c5f0f00 100644
--- a/src/bettola.cpp
+++ b/src/bettola.cpp
@@ -7,6 +7,7 @@
 #include <SDL3/SDL_video.h>
 #include <cstdlib>
 #include <cstdio>
+#include "bettola/math/vec3.h"
 #include "game/player.h"
 #include "graphics/camera.h"
 
diff --git a/src/game/remote_player.cpp b/src/game/remote_player.cpp
index e0daf3e..1cd1909 100644
--- a/src/game/remote_player.cpp
+++ b/src/game/remote_player.cpp
@@ -1,20 +1,21 @@
 #include "remote_player.h"
 
-RemotePlayer::RemotePlayer(unsigned int id, float x, float y) :
-    PlayerBase(), _target_x(x), _target_y(y), _target_yaw(0.0f) {
+RemotePlayer::RemotePlayer(unsigned int id, float x, float y, float z) :
+    PlayerBase(), _target_x(x), _target_y(y), _target_z(z), _target_yaw(0.0f) {
   _id = id; /* Manually set id from the server. */
-  _position = {x, 0.0f, y };
+  _position = {x, y, z};
 }
 
 void RemotePlayer::update(double dt) {
   const float interp_speed = 15.0f;
 
   _position.x += (_target_x - _position.x) * interp_speed * dt;
-  _position.z += (_target_y - _position.z) * interp_speed * dt; /* y is z. */
+  _position.y += (_target_y - _position.y) * interp_speed * dt;
+  _position.z += (_target_z - _position.z) * interp_speed * dt;
   /* TODO: Snap the yaw, we'll interpolate later if we need. */
   _yaw = _target_yaw;
 }
 
-void RemotePlayer::set_target_position(float x, float y, float yaw) {
-  _target_x = x; _target_y = y; _target_yaw = yaw;
+void RemotePlayer::set_target_position(float x, float y, float z, float yaw) {
+  _target_x = x; _target_y = y; _target_z = z; _target_yaw = yaw;
 }
diff --git a/src/game/remote_player.h b/src/game/remote_player.h
index 8ec23b5..029864b 100644
--- a/src/game/remote_player.h
+++ b/src/game/remote_player.h
@@ -4,13 +4,14 @@
 
 class RemotePlayer : public PlayerBase {
 public:
-  RemotePlayer(unsigned int id, float x, float y);
+  RemotePlayer(unsigned int id, float x, float y, float z);
 
   void update(double dt);
-  void set_target_position(float x, float y, float yaw);
+  void set_target_position(float x, float y, float z, float yaw);
 
 private:
   float _target_x;
   float _target_y;
+  float _target_z;
   float _target_yaw;
 };
diff --git a/src/game/world.cpp b/src/game/world.cpp
index 84dbf90..11e0075 100644
--- a/src/game/world.cpp
+++ b/src/game/world.cpp
@@ -1,6 +1,7 @@
 #include "world.h"
 #include <cstring>
 #include "bettola/game/chunk.h"
+#include "bettola/game/terrain.h"
 #include "graphics/chunk_mesh.h"
 
 World::~World(void) {
@@ -10,7 +11,7 @@ World::~World(void) {
 }
 
 void World::add_chunk(int chunk_x, int chunk_z, const float* heightmap) {
-  ChunkPos pos = { chunk_x, chunk_z };
+  ChunkPos pos = {chunk_x, chunk_z};
   if(_chunk_meshes.count(pos)) {
     /* TODO: Chunk already exists, perhaps update it later.. */
     return;
@@ -19,5 +20,24 @@ void World::add_chunk(int chunk_x, int chunk_z, const float* heightmap) {
   BettolaLib::Game::Chunk chunk;
   memcpy(chunk.heightmap.data(), heightmap, chunk.heightmap.size()*sizeof(float));
 
+  _chunks[pos] = chunk;
   _chunk_meshes[pos] = new ChunkMesh(chunk_x, chunk_z, chunk);
 }
+
+float World::get_height(float world_x, float world_z) const {
+  float chunk_width   = (float)(BettolaLib::Game::CHUNK_WIDTH -1);
+  float chunk_height  = (float)(BettolaLib::Game::CHUNK_HEIGHT-1);
+
+  int chunk_x = floor(world_x / chunk_width);
+  int chunk_z = floor(world_z / chunk_height);
+
+  float local_x = world_x - (chunk_x * chunk_width);
+  float local_z = world_z - (chunk_z * chunk_height);
+
+  auto it = _chunks.find({chunk_x, chunk_z});
+  if(it == _chunks.end()) {
+    return 0.0f; /* No chunk data. */
+  }
+
+  return BettolaLib::Game::Terrain::get_height_at(it->second, local_x, local_z);
+}
diff --git a/src/game/world.h b/src/game/world.h
index d4562c1..0b023e9 100644
--- a/src/game/world.h
+++ b/src/game/world.h
@@ -2,6 +2,7 @@
 
 #include <map>
 
+#include "bettola/game/chunk.h"
 #include "graphics/chunk_mesh.h"
 
 /* Allow the use of a pair of ints as a map key. */
@@ -18,6 +19,7 @@ class World {
 public:
   ~World(void);
 
+  float get_height(float world_x, float world_z) const;
   void add_chunk(int chunk_x, int chunk_z, const float* heightmap);
   const std::map<ChunkPos, ChunkMesh*>& get_chunk_meshes(void) const {
     return _chunk_meshes;
@@ -25,4 +27,5 @@ public:
 
 private:
   std::map<ChunkPos, ChunkMesh*> _chunk_meshes;
+  std::map<ChunkPos, BettolaLib::Game::Chunk> _chunks;
 };
diff --git a/src/game_client.cpp b/src/game_client.cpp
index 6be57ca..2887221 100644
--- a/src/game_client.cpp
+++ b/src/game_client.cpp
@@ -129,7 +129,7 @@ void GameClient::_process_game_state(const BettolaLib::Network::GameStateMessage
 
     if(ps.player_id == _our_player_id) {
       /* This is our player. Reconcile. */
-      _player.set_position({ps.x, 0.0f, ps.y});
+      _player.set_position({ps.x, ps.y, ps.z});
 
       /* Remove all inputs from our history that the server has processed. */
       while(!_pending_inputs.empty() &&
@@ -140,8 +140,12 @@ void GameClient::_process_game_state(const BettolaLib::Network::GameStateMessage
       /* Now, re-apply any remaining inputs that the server hasn't
        * seen yet. This'll bring the client up to date with most recent inputs.
        */
-      for(const auto& input : _pending_inputs) {
-        _player.apply_input(input);
+      for(const auto& input_msg : _pending_inputs) {
+        PlayerBase::InputState input_state = { input_msg.up, input_msg.down,
+          input_msg.left, input_msg.right };
+        BettolaMath::Vec3 cam_front = { input_msg.cam_front_x, input_msg.cam_front_y,
+          input_msg.cam_front_z };
+        _player.set_velocity_direction(input_state, cam_front);
       }
     } else {
       /* Remote player. Find if we already know about them.. */
@@ -151,10 +155,10 @@ void GameClient::_process_game_state(const BettolaLib::Network::GameStateMessage
 
       if(it != _remote_players.end()) {
         /* Found 'em! update their target pos. */
-        it->set_target_position(ps.x, ps.y, ps.yaw);
+        it->set_target_position(ps.x, ps.y, ps.z, ps.yaw);
       } else {
         /* They are new, add them to our list. */
-        _remote_players.emplace_back(ps.player_id, ps.x, ps.y);
+        _remote_players.emplace_back(ps.player_id, ps.x, ps.y, ps.z);
       }
     }
   }
@@ -197,6 +201,9 @@ void GameClient::send_input(PlayerBase::InputState& input,
 
 void GameClient::update_players(float dt) {
   _player.update(dt);
+  float terrain_height = _world.get_height(_player.get_position().x,
+                                           _player.get_position().z);
+  _player.apply_gravity_and_collision(terrain_height);
   for(auto& p : _remote_players) {
     p.update(dt);
   }
diff --git a/srv/game/game.cpp b/srv/game/game.cpp
index 6ddef26..3d36b78 100644
--- a/srv/game/game.cpp
+++ b/srv/game/game.cpp
@@ -53,6 +53,10 @@ void Game::process_udp_message(const char* buffer, size_t size, const sockaddr_i
       BettolaMath::Vec3 cam_front = { msg.cam_front_x, msg.cam_front_y, msg.cam_front_z };
       player->set_velocity_direction(input, cam_front);
       player->update(msg.dt);
+
+      float terrain_height = _world.get_height(player->get_position().x,
+                                               player->get_position().z);
+      player->apply_gravity_and_collision(terrain_height);
     }
   }
 }
@@ -66,7 +70,8 @@ void Game::broadcast_game_state(BettolaLib::Network::UDPSocket& udp_socket) {
     msg.players[i].player_id = _players[i]->get_id();
     const auto& pos     = _players[i]->get_position();
     msg.players[i].x    = pos.x;
-    msg.players[i].y    = pos.z; /* Send z as y for the 2D style network message. */
+    msg.players[i].y    = pos.y;
+    msg.players[i].z    = pos.z;
     msg.players[i].yaw  = _players[i]->get_yaw();
     msg.players[i].last_processed_sequence = _players[i]->get_last_processed_sequence();
   }
diff --git a/srv/game/world.h b/srv/game/world.h
index bfd3784..c139e23 100644
--- a/srv/game/world.h
+++ b/srv/game/world.h
@@ -18,6 +18,7 @@ public:
   World(void);
 
   BettolaLib::Game::Chunk& get_chunk(int x, int z);
+  float get_height(float world_x, float world_z);
 
 private:
   void _generate_chunk(BettolaLib::Game::Chunk& chunk, int chunk_x, int chunk_z);
diff --git a/srv/game/world_collision.cpp b/srv/game/world_collision.cpp
new file mode 100644
index 0000000..e3e3adc
--- /dev/null
+++ b/srv/game/world_collision.cpp
@@ -0,0 +1,23 @@
+#include <cmath>
+#include "world.h"
+#include "bettola/game/chunk.h"
+#include "bettola/game/terrain.h"
+
+float World::get_height(float world_x, float world_z) {
+  /*
+   * Mesh generation uses (WIDTH-1), so account for it
+   * when calculating the chunk and local coords.
+   */
+  float chunk_width   = (float)(BettolaLib::Game::CHUNK_WIDTH-1);
+  float chunk_height  = (float)(BettolaLib::Game::CHUNK_HEIGHT-1);
+
+  int chunk_x = floor(world_x / chunk_width);
+  int chunk_z = floor(world_z / chunk_height);
+
+  float local_x = world_x - (chunk_x * chunk_width);
+  float local_z = world_z - (chunk_z * chunk_height);
+
+  BettolaLib::Game::Chunk& chunk = get_chunk(chunk_x, chunk_z);
+
+  return BettolaLib::Game::Terrain::get_height_at(chunk, local_x, local_z);
+}