Mark Wolf restricts the definition to games featuring multiple antagonists (''em' being short for 'them'), calling games featuring one-on-one shooting 'combat games'. 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.

Some restrict the genre to games featuring some kind of craft, using fixed or scrolling movement. Beyond this, critics differ on exactly which design elements constitute a shoot 'em up. The controlling player must rely primarily on reaction times to succeed. 3.5 Bullet hell and niche appeal (mid-1990s to present)ДЄ '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.

3.4 Run-and-gun and rail shooters (1980s to early 1990s).

3.3 Golden age and refinement (late 1970s to early 1980s).

3.2 Emergence of shoot 'em up genre (late 1970s).

'Bullet hell' games are a subgenre of shooters that features overwhelming numbers of enemy projectiles, often in visually impressive formations. 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. Shoot 'em ups were popular throughout the 1980s to early 1990s, diversifying into a variety of subgenres such as scrolling shooters, run-and-gun games and rail shooters. The genre was then further developed by arcade hits such as Asteroids and Galaxian in 1979.

The shoot 'em up genre was established by the hit arcade game Space Invaders, which popularised and set the general template for the genre in 1978, and spawned many clones. The genre's roots can be traced back to earlier shooting games, including target shooting electro-mechanical games of the mid-20th-century and the early mainframe game Spacewar! (1962). 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, while others allow a broader definition including characters on foot and a variety of perspectives. Shoot 'em ups (also known as shmups or STGs ) are a sub-genre of shooter video games, which are in turn a sub-genre of action video games.

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