Shoot 'em up (also known as shmup or STG) is a subgenre of shooter games. In a shoot 'em up, the player controls a lone character, often in a spacecraft or aircraft, shooting large numbers of enemies while dodging their attacks. The genre in turn encompasses various types or subgenres and critics differ on exactly what design elements constitute a shoot 'em up. Some restrict the definition to games featuring spacecraft and certain types of character movement; others allow a broader definition including characters on foot and a variety of perspectives. Shoot 'em ups call for fast reactions and for the player to memorise levels and enemy attack patterns. Newer "bullet hell" games feature overwhelming numbers of enemy projectiles.
The genre's origins can be traced back to Spacewar!, one of the very earliest computer games, developed in 1961 and eventually released in arcades in the early 1970s. However, Space Invaders, released in Japanese arcades in 1978, is generally credited with inventing and popularising the genre proper. Shoot 'em ups were popular throughout the 1980s and early 1990s as they evolved. From the mid-1990s and the burgeoning use of 3D graphics in video games, shoot 'em ups became a niche genre based on design conventions established in the 1980s and increasingly catered to specialist enthusiasts, particularly in Japan.
Shoot 'em ups encompass various types, or subgenres. In a "fixed shooter" such as Space Invaders, the protagonist can only move across one axis and enemies attack from a single direction. In a "multidirectional shooter" the protagonist may rotate and move in any direction. By contrast, a "rail shooter" protagonist is viewed from behind and moves "into the screen", while the player retains control over dodging. "Tube shooters" feature similar viewpoints, and their protagonists fly through abstract tubes. "Scrolling shooters" encompass both horizontal and vertical perspectives. "Run and gun" games feature protagonists on foot, rather than spacecraft, that often have the ability to jump; they may feature either scrolling or multidirectional movement.