Counter-Strike (CS) is a series of multiplayer tactical first-person shooter video games in which teams of terrorists battle to perpetrate an act of terror (bombing, hostage-taking, assassination) while counter-terrorists try to prevent it (bomb defusal, hostage rescue, escort mission). The series began on Windows in 1999 with the release of the first game, Counter-Strike. It was initially released as a modification ("mod") for Half-Life that was designed by Minh "Gooseman" Le and Jess "Cliffe" Cliffe before the rights to the mod's intellectual property were acquired by Valve, the developers of Half-Life, who then turned Counter-Strike into a retail product released in 2000.

The original Counter-Strike was followed by Counter-Strike: Condition Zero, developed by Turtle Rock Studios and released in March 2004. A previous version of Condition Zero that was developed by Ritual Entertainment was released alongside it as Condition Zero: Deleted Scenes. Eight months later, Valve released Counter-Strike: Source, a remake of the original Counter-Strike and the first in the series to run on Valve's then-newly created Source engine.[4] 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 worked on Counter-Strike: Source post-release, helped to develop the game alongside Valve.[5] Counter-Strike 2 was announced in March 2023 and publicly released on September 27, 2023, as a replacement for Global Offensive.

There have been several third-party spin-off titles created for Asian markets over the years. These include the Counter-Strike Online series, Counter-Strike Neo, and Counter-Strike Nexon: Studio.