diff --git a/README.md b/README.md
index d3869007..e1c6943d 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,6 @@ This project is primarily for my own nostalgia; I offer no guarantees on how or
 
 Current known issues / missing features:
 - Test all the communication features in cross-version scenarios (info board, simple mail, card search, etc.)
-- The trade window isn't implemented.
 - Episode 3 battles aren't implemented.
 - PSO PC and PSOBB are not well-tested and likely will disconnect when clients try to use unimplemented features. Only GC is known to be stable and mostly complete.
 - Patches currently are platform-specific but not version-specific. This makes them quite a bit harder to write and use properly.
diff --git a/src/CommandFormats.hh b/src/CommandFormats.hh
index 158ccf85..a7661fdd 100644
--- a/src/CommandFormats.hh
+++ b/src/CommandFormats.hh
@@ -1700,24 +1700,53 @@ struct S_Unknown_GC_Ep3_CC {
 // CE: Invalid command
 // CF: Invalid command
 
-// D0 (C->S): Execute trade via trade window (GC/BB)
-// General sequence: client sends D0, server sends D1 to that client, server
-// sends D3 to other client, server sends D4 to both (?) clients.
-// Format unknown. On PSO GC it appears to be always 0x288 bytes in size; on BB
-// it is 0x28C bytes in size, implying that the format is the same between the
-// two versions (since BB headers are 4 bytes longer).
+// D0 (C->S): Start trade sequence (GC/BB)
+// The trade window sequence is a bit complicated. The normal flow is:
+// - Clients sync trade state with 60xA6 commands (technically 62xA6)
+// - When both have confirmed, one client (the initiator) sends a D0
+// - Server sends a D1 to the non-initiator
+// - Non-initiator sends a D0
+// - Server sends a D1 to both clients
+// - Both clients delete the sent items from their inventories (and send the
+//   appropriate subcommand)
+// - Both clients send a D2 (similarly to how AC works, the server should not
+//   proceed until both D2s are received)
+// - Server sends a D3 to both clients with each other's data from their D0s,
+//   followed immediately by a D4 01 to both clients, which completes the trade
+// - Both clients send the appropriate subcommand to create inventory items
+// TODO: On BB, is the server responsible for sending the appropriate item
+// subcommands?
+// At any point if an error occurs, either client may send a D4 00, which
+// cancels the entire sequence. The server should then send D4 00 to both
+// clients.
 
-// D1 (S->C): Confirm trade to initiator (GC/BB)
+struct SC_TradeItems_D0_D3 { // D0 when sent by client, D3 when sent by server
+  le_uint16_t target_client_id;
+  le_uint16_t item_count;
+  // Note: PSO GC sends uninitialized data in the unused entries of this
+  // command. newserv parses and regenerates the item data when sending D3,
+  // which effectively erases the uninitialized data.
+  ItemData items[0x20];
+};
+
+// D1 (S->C): Advance trade state (GC/BB)
 // No arguments
+// See D0 description for usage information.
 
-// D2 (C->S): Unknown (used in trade sequence)
+// D2 (C->S): Trade can proceed (GC/BB)
 // No arguments
+// See D0 description for usage information.
 
-// D3 (S->C): Execute trade with accepter (GC/BB)
-// Format unknown; appears to be same as D0.
+// D3 (S->C): Execute trade (GC/BB)
+// Same format as D0. See D0 description for usage information.
 
-// D4 (S->C): Close trade (GC/BB)
+// D4 (C->S): Trade failed (GC/BB)
 // No arguments
+// See D0 description for usage information.
+
+// D4 (S->C): Trade complete (GC/BB)
+// header.flag must be 0 (trade failed) or 1 (trade complete).
+// See D0 description for usage information.
 
 // D5: Large message box (GC/BB)
 // Same as 1A command, except the maximum length of the message is 0x1000 bytes.
diff --git a/src/Player.hh b/src/Player.hh
index e3888fb3..a9af7a06 100644
--- a/src/Player.hh
+++ b/src/Player.hh
@@ -83,6 +83,14 @@ struct PlayerBank { // 0xFA8 bytes
 
 
 
+struct PendingItemTrade {
+  uint8_t other_client_id;
+  bool confirmed; // true if client has sent a D2 command
+  std::vector<ItemData> items;
+};
+
+
+
 struct PlayerDispDataBB;
 
 // PC/GC player appearance and stats data
@@ -437,6 +445,9 @@ public:
 
   // The following fields are not saved, and are only used in certain situations
 
+  // Null unless the client is within the trade sequence (D0-D4 commands)
+  std::unique_ptr<PendingItemTrade> pending_item_trade;
+
   // Null unless the client is Episode 3 and has sent its config already
   std::shared_ptr<Ep3Config> ep3_config;
 
diff --git a/src/ReceiveCommands.cc b/src/ReceiveCommands.cc
index 58f01f38..a1564708 100644
--- a/src/ReceiveCommands.cc
+++ b/src/ReceiveCommands.cc
@@ -2019,6 +2019,109 @@ void process_client_ready(shared_ptr<ServerState> s, shared_ptr<Client> c,
   }
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// Trade window commands
+
+void process_trade_start(shared_ptr<ServerState> s, shared_ptr<Client> c,
+    uint16_t, uint32_t, const string& data) { // D0
+  auto& cmd = check_size_t<SC_TradeItems_D0_D3>(data);
+
+  if (c->game_data.pending_item_trade) {
+    throw runtime_error("player started a trade when one is already pending");
+  }
+  if (cmd.item_count > 0x20) {
+    throw runtime_error("invalid item count in trade items command");
+  }
+
+  auto l = s->find_lobby(c->lobby_id);
+  if (!l || !l->is_game()) {
+    throw runtime_error("trade command received in non-game lobby");
+  }
+  auto target_c = l->clients.at(cmd.target_client_id);
+  if (!target_c) {
+    throw runtime_error("trade command sent to missing player");
+  }
+
+  c->game_data.pending_item_trade.reset(new PendingItemTrade());
+  c->game_data.pending_item_trade->other_client_id = cmd.target_client_id;
+  for (size_t x = 0; x < cmd.item_count; x++) {
+    c->game_data.pending_item_trade->items.emplace_back(cmd.items[x]);
+  }
+
+  // If the other player has a pending trade as well, assume this is the second
+  // half of the trade sequence, and send a D1 to both clients (which should
+  // cause them to delete the appropriate inventory items and send D2s). If the
+  // other player does not have a pending trade, assume this is the first half
+  // of the trade sequence, and send a D1 only to the target player (to request
+  // its D0 command).
+  send_command(target_c, 0xD1, 0x00);
+  if (target_c->game_data.pending_item_trade) {
+    send_command(c, 0xD1, 0x00);
+  }
+}
+
+void process_trade_execute(shared_ptr<ServerState> s, shared_ptr<Client> c,
+    uint16_t, uint32_t, const string& data) { // D2
+  check_size_v(data.size(), 0);
+
+  if (!c->game_data.pending_item_trade) {
+    throw runtime_error("player executed a trade with none pending");
+  }
+
+  auto l = s->find_lobby(c->lobby_id);
+  if (!l || !l->is_game()) {
+    throw runtime_error("trade command received in non-game lobby");
+  }
+  auto target_c = l->clients.at(c->game_data.pending_item_trade->other_client_id);
+  if (!target_c) {
+    throw runtime_error("target player is missing");
+  }
+  if (!target_c->game_data.pending_item_trade) {
+    throw runtime_error("player executed a trade with no other side pending");
+  }
+
+  c->game_data.pending_item_trade->confirmed = true;
+  if (target_c->game_data.pending_item_trade->confirmed) {
+    send_execute_item_trade(c, target_c->game_data.pending_item_trade->items);
+    send_execute_item_trade(target_c, c->game_data.pending_item_trade->items);
+    send_command(c, 0xD4, 0x01);
+    send_command(target_c, 0xD4, 0x01);
+    c->game_data.pending_item_trade.reset();
+    target_c->game_data.pending_item_trade.reset();
+  }
+}
+
+void process_trade_error(shared_ptr<ServerState> s, shared_ptr<Client> c,
+    uint16_t, uint32_t, const string& data) { // D4
+  check_size_v(data.size(), 0);
+
+  // Annoyingly, if the other client disconnects at a certain point during the
+  // trade sequence, the client can get into a state where it sends this command
+  // many times in a row. To deal with this, we just do nothing if the client
+  // has no trade pending.
+  if (!c->game_data.pending_item_trade) {
+    return;
+  }
+  uint8_t other_client_id = c->game_data.pending_item_trade->other_client_id;
+  c->game_data.pending_item_trade.reset();
+  send_command(c, 0xD4, 0x00);
+
+  // Cancel the other side of the trade too, if it's open
+  auto l = s->find_lobby(c->lobby_id);
+  if (!l || !l->is_game()) {
+    throw runtime_error("trade command received in non-game lobby");
+  }
+  auto target_c = l->clients.at(other_client_id);
+  if (!target_c) {
+    return;
+  }
+  if (!target_c->game_data.pending_item_trade) {
+    return;
+  }
+  target_c->game_data.pending_item_trade.reset();
+  send_command(target_c, 0xD4, 0x00);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Team commands
 
@@ -2342,8 +2445,8 @@ static process_command_t gc_handlers[0x100] = {
   nullptr, nullptr, nullptr, nullptr,
 
   // D0
-  nullptr, nullptr, nullptr, nullptr, // D0 is process trade
-  nullptr, nullptr, process_message_box_closed, process_gba_file_request,
+  process_trade_start, nullptr, process_trade_execute, nullptr,
+  process_trade_error, nullptr, process_message_box_closed, process_gba_file_request,
   process_info_board_request, process_write_info_board_t<char>, nullptr, process_verify_license_gc,
   process_ep3_menu_challenge, nullptr, nullptr, nullptr,
 
@@ -2431,8 +2534,8 @@ static process_command_t bb_handlers[0x100] = {
   nullptr, nullptr, nullptr, nullptr,
 
   // D0
-  nullptr, nullptr, nullptr, nullptr,
-  nullptr, nullptr, nullptr, nullptr,
+  process_trade_start, nullptr, process_trade_execute, nullptr,
+  process_trade_error, nullptr, nullptr, nullptr,
   process_info_board_request, process_write_info_board_t<char16_t>, nullptr, nullptr,
   process_guild_card_data_request_bb, nullptr, nullptr, nullptr,
 
diff --git a/src/SendCommands.cc b/src/SendCommands.cc
index 9e1afe60..462db23d 100644
--- a/src/SendCommands.cc
+++ b/src/SendCommands.cc
@@ -1092,6 +1092,25 @@ void send_get_player_info(shared_ptr<Client> c) {
 
 
 
+////////////////////////////////////////////////////////////////////////////////
+// Trade window
+
+void send_execute_item_trade(std::shared_ptr<Client> c,
+    const std::vector<ItemData>& items) {
+  SC_TradeItems_D0_D3 cmd;
+  if (items.size() > sizeof(cmd.items) / sizeof(cmd.items[0])) {
+    throw logic_error("too many items in execute trade command");
+  }
+  cmd.target_client_id = c->lobby_client_id;
+  cmd.item_count = items.size();
+  for (size_t x = 0; x < items.size(); x++) {
+    cmd.items[x] = items[x];
+  }
+  send_command_t(c, 0xD3, 0x00, cmd);
+}
+
+
+
 ////////////////////////////////////////////////////////////////////////////////
 // arrows
 
diff --git a/src/SendCommands.hh b/src/SendCommands.hh
index e8acb555..8d2c311b 100644
--- a/src/SendCommands.hh
+++ b/src/SendCommands.hh
@@ -183,6 +183,9 @@ void send_player_leave_notification(std::shared_ptr<Lobby> l,
 void send_self_leave_notification(std::shared_ptr<Client> c);
 void send_get_player_info(std::shared_ptr<Client> c);
 
+void send_execute_item_trade(std::shared_ptr<Client> c,
+    const std::vector<ItemData>& items);
+
 void send_arrow_update(std::shared_ptr<Lobby> l);
 void send_resume_game(std::shared_ptr<Lobby> l,
     std::shared_ptr<Client> ready_client);