Counter-Strike, commonly abbreviated to CS, is a team-based, tactical first-person shooter game which originated with a total conversion mod created by Minh "gooseman" Le and Jess Cliffe, of Valve Software’s first-person shooter, Half-Life. The game has been expanded into a series since its original release, which currently includes Counter-Strike: Condition Zero and Counter-Strike: Source.

Counter-Strike pits a team of counter-terrorists against a team of terrorists in rounds of competition won by completing an objective or eliminating the opposing force. The latest incarnation of the game, Counter-Strike: Source, is based on the Source engine developed for Half-Life 2. Counter-Strike is widely acknowledged as the most successful and popular of the tactical shooter genre. Signs of Counter-Strike’s wide influence can be found in mods for games such as Quake III Arena, Unreal Tournament, and other standalone shooters such as Global Operations, Americas Army, Call of Duty and many more.

As of May 2006, Counter-Strike is still the most widely played online first-person shooter in the world. In 2002 there were over 30,000 Counter-Strike servers on the Internet (second place was Unreal Tournament with about 9,800). In 2004, GameSpy statistics showed over 85,000 players simultaneously playing Counter-Strike at any point in time, and in 2006, Steam regularly shows over 200,000 players for Counter-Strike (this includes Counter-Strike: Source, Counter-Strike: Condition Zero and Counter-Strike). According to statistics gathered by Valve’s content-delivery platform, Steam, these players contribute to over 4.5 billion minutes of playing time each month, solidifying its position as the most popular online first-person shooter in history. Counter-Strike was originally played online through the WON gaming service, which was shut down in 2004, forcing players to switch to Steam (to which a section of players responded by creating their own WON network, dubbed WON2).

Counter-Strike is a team-based, first-person shooter in which players join either the terrorist or counter-terrorist team, and combat the opposing team while fulfilling predetermined objectives. Server settings may automatically balance teams when one team has more players than the other. Each round starts with the two teams spawning simultaneously, as one of eight different default character models (four to choose from for each the counter-terrorist and terrorist teams. Counter-Strike: Condition Zero added two extra models, bringing the total to ten). Each player generally starts with $800, two magazines of ammunition, a knife, and a pistol: a Heckler & Koch USP .45 Tactical for counter-terrorists, and a GLOCK 18c for terrorists. Players are generally given a few seconds before the round begins (known as "freeze time") to buy equipment, during which they cannot attack or move in any direction. Players may buy equipment whenever they are in a "buy zone" for their team (some zones can be for both teams, depending on the map) provided the round has not been in session for longer than a specified time (90 seconds is default). Once the round has ended, surviving players retain their equipment for use in the next round; those who have died begin the next round with the default pistol and knife.