**Tremulous** is a free and open source team-based first-person shooter with real-time strategy elements.

The game features two opposing teams: [[tremulous:knowledgebase:humans:|Humans]] and [[tremulous:knowledgebase:aliens|Aliens]]. Each team must attack the enemy's base and team members while defending their own base. 

**Gameplay**

During a match, players may attack the enemy, or maintain the base and construct new structures. Humans utilize various weapons, armor and other upgrades, while aliens may evolve into more powerful classes, each of which possesses unique abilities.

The teams do not have access to all possible upgrades and buildings at the beginning of the game. Each team must reach a threshold of frags in order to advance to the next developmental stage. The point at which a stage change occurs varies dynamically depending on the number of players on each team. Each new stage brings more upgrades for the humans and more classes for the aliens.

In addition to requiring a certain stage, each human item or alien class must be purchased using currency earned in game. The aliens are awarded frags for killing their foes which may be used to evolve (these points are commonly called evos by players). The Humans gain credits. Players earn currency by killing enemy players or by destroying certain enemy structures; the amount of received currency depends on what class the opponent was (or what structure was destroyed) and how much of the target's total damage was dealt by the killing player. As a mechanism to encourage builders, who naturally don't engage in combat, players are also rewarded currency every two minutes by simply staying alive.

Builders are responsible for construction of new buildings and base maintenance. Each building takes up a certain number of build points available to each team. The number of build points can vary from map to map and server to server, limiting how large bases can become. To reduce the likelihood of the match ending in a tie, most games enter a sudden death mode ten to fifteen minutes before the draw/tie time limit. This is a period during which the building of most or all structures is prohibited, allowing attackers to more easily overwhelm the defenders.

**Devlopment**

Tremulous development commenced in early 2000 as a modification for the commercial computer game Quake III Arena. The gameplay is generally inspired by the Quake II modification Gloom, although they share no content. Version 1.0.0 was released on August 11, 2005.

Though Tremulous features a similar theme and game-play to Natural Selection,the developers say it is neither based on it nor inspired by it. According to the developers, development on the game began "long before NS was in the public domain".

Following the release of the Quake III Arena source code under the GPLv2 on August 19, 2005, the developers decided to rework Tremulous into a standalone, free and open source game. The current stable version, 1.1.0, was released on March 31, 2006 and is based upon ioquake3, a modified id Tech 3 engine. This version was better received than previous versions and attracted a considerable following in comparison to its peers.

Tremulous is licensed under the GNU GPL (v2+) license, although it includes a few libraries released under other GPL-compatible licenses. Most of the game media is licensed under the Creative Commons Attribution-ShareAlike 2.5 License.

There are clients for playing Tremulous available for Windows and GNU/Linux, and an unofficial client for Mac OS X(for version 1.1). An official client is available for 1.2 Beta for Mac. The release of the game as free and open source software allows programmers to modify the game. A growing number of players are releasing patches for the game in order to remove bugs, add features to the game or modify the gameplay. Tony J. White (tjw), a key developer of the ETPub mod for Wolfenstein: Enemy Territory, contributed an administrative system and a backported client that were soon incorporated into the official Subversion repository.

The latest development version is 1.2 beta "Gameplay Preview". It was released on December 4, 2009 for community testing and review. It has introduced a large number of gameplay as well as server and client changes and upgrades.