Half-Life is a series of First Person Shooter video game in a science fiction atmosphere. This series is composed of two full games, Half-Life and Half-Life 2 with five expansions each.

The first opus of this series was created in 1998, it was and it still is developed by Valve Software and it is published by Sierra Studios. The game starts a few minutes before the accident of Black Mesa; you play Gordon Freeman one of the scientists present during the experience that opened a portal allowing aliens to enter the base. Gordon now has to explore the base, shooting every alien on his way, in order to remedy the dramatic situation he is into.

The expansions of this opus are Half-Life: Uplink (1999), Half-Life: Opposing Force (1999), Half-Life Generation (2000), Half-Life: Decay (2001) and Half-Life: Blue Shift (2001). In those expansions you can play Gordon Freeman, Adrian Shepard, or Barney Calhoun.

In Half-Life 2 (2004) and in its expansions you only control Gordon Freeman. The story takes place 20 years after Half-Life and the game is known for its game engine in which every object had its own physics engine and collision detection.

This game completely changed the way FPS was made and it became a reference for the many upcoming FPS.