A computer / video game where points are scored or progress is made by shooting projectiles at enemies. This is about the only common element between most shoot 'em ups, as the entity you control, and the style in which the world is navigated vary widely.

When the term is used on it's own by the gaming press or most gamers, it usually refers to games where you control a space ship, and where the screen scrolls either horizontally (eg, Gradius, R-Type), or vertically (eg, Gunhed, Xevious, Aleste, 1942), and also top down shoot 'em ups (eg, Ikari Warriors, Mercs), where you control a character viewed from above who can shoot and move freely in eight directions.

In recent years, the first person shoot 'em up (FPS) has become prevalant, and the traditional shoot 'em up has fallen to the wayside, only really retaining popularity in Japan and in the arcades. FPS games are viewed through the eyes of the character, and (usually) full freedom of movement is given. When the phrase "shoot 'em up" is used in the mainstream press, it more often than not refers to FPS games, unless it is in the context of classic gaming.

There is debate as to whether platform shoot 'em ups (eg, Metal Slug, Gunstar Heroes, Contra, Assault Suits Valken) should be filed under platformer or shoot 'em up, being simplistic platformers with a heavy emphasis on shooting stuff. The truth is that they fit comfortably into both genres.

There are also many examples of shoot ‘em ups that don’t fit into the styles described above yet still fit the defining criterea. Though the one exception is light gun games, which are generally given their own category.

See also: shmups