is a series of multiplayer first-person shooter video games, in which teams of terrorists battle to perpetrate an act of terror (bombing, hostage-taking) and counter-terrorists try to prevent it (bomb defusal, hostage rescue). The series began on Windows in 1999 with the first game, Counter-Strike. It was initially released as a modification ("mod") for Half-Life and designed by Minh "Gooseman" Le and Jess "Cliffe" Cliffe before the rights to the game's intellectual property were acquired by Valve Corporation, the developers of Half-Life.
That original game was followed by Counter-Strike: Condition Zero, developed by Turtle Rock Studios and released in 2004. In November of that year, Counter-Strike: Source was released by Valve. Released only eight months after Condition Zero, Source was a remake of the original Counter-Strike and the first in the series to run on Valve's newly created Source engine. The fourth game in the main series, Counter-Strike: Global Offensive, was released by Valve in 2012 for Windows, OS X, Xbox 360, and PlayStation 3. Hidden Path Entertainment, who also worked on Counter-Strike: Source post-release, helped to develop the game alongside Valve.
There have been several community-made spin-off titles created over the years. These include the Online series, Neo, and Nexon: Zombies.