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Shoot 'em up (also known as shmup or STG) is a subgenre of the shooter genre of video games. In a shoot 'em up, the player character engages in a lone assault, often in a spacecraft or aircraft, shooting large numbers of enemies while dodging their attacks. There is no consensus as to which design elements compose 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 memorize levels and enemy attack patterns. "Bullet hell" games feature overwhelming numbers of enemy projectiles.

The genre's origins can be traced back to Spacewar!, one of the earliest computer games, developed in 1962 and eventually released in amusement arcades in the early 1970s. Later in 70s, games such as Space Invaders and Asteroids popularized the genre. Shoot 'em ups were popular throughout the 1980s and early 1990s. In the mid-1990s, shoot 'em ups became a niche genre based on design conventions established in the 1980s, and increasingly catered to specialist enthusiasts, particularly in Japan.

A "shoot 'em up", also known as a "shmup" or "STG" (the common Japanese abbreviation for "shooting games"), is a game in which the protagonist combats a large number of enemies by shooting at them while dodging their fire. The controlling player must rely primarily on reaction times to succeed. Beyond this, critics differ on exactly which design elements constitute a shoot 'em up. Some restrict the genre to games featuring some kind of craft, using fixed or scrolling movement. Others widen the scope to include games featuring such protagonists as robots or humans on foot, as well as including games featuring "on-rails" (or "into the screen") and "run and gun" movement. Mark Wolf restricts the definition to games featuring multiple antagonists ("'em" being short for "them"), calling games featuring one-on-one shooting "combat games". Formerly, critics described any game where the primary design element was shooting as a "shoot 'em up", but later shoot 'em ups became a specific, inward-looking genre based on design conventions established in those shooting games of the 1980s.

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