Shoot 'em Ups, also known as shmups or STGs (the common Japanese abbreviation for "shooting games") are a sub-genre of action games. A game of the genre is that 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.

     Space Invaders (1978) is most frequently cited as the "first" or "original" in the genre. A seminal game created by Tomohiro Nishikado of Japan's Taito, it led to proliferation of shooter games. It pitted the player against multiple enemies descending from the top of the screen at a constantly increasing speed. Nishikado conceived the game by combining elements of Breakout (1976) with those of earlier target shooting games, and simple alien creatures inspired by H. G. Wells' The War of the Worlds.

     Following the success of Space Invaders, shoot 'em ups became the dominant genre for much of the golden age of arcade video games, from the late 1970s up until the early 1980s, particularly the "space shooter" subgenre. In 1979, Namco's Galaxian—"the granddaddy of all top-down shooters", according to IGN—was released. Its use of colour graphics and individualised antagonists were considered "strong evolutionary concepts" among space ship games. In 1981 Gorf brought joystick control and (limited) vertical as well as hoizontal movement to the vertically-oriented fixed-shooter genre, while Space Invaders and Galaxian have only horizontal movement controlled by a pair of buttons. Atari's Asteroids (1979) was a hit multi-directional shooter, taking from Spacewar! the ability for the player's ship to roam the entire screen and to rotate, move and shoot in any direction.

     The Space Invaders format evolved into the vertical scrolling shooter sub-genre. SNK's debut shoot 'em up Ozma Wars (1979) featured vertical scrolling backgrounds and enemies, and it was the first action game to feature a supply of energy, similar to hit points. Namco's Xevious, released in 1982, was one of the first and most influential vertical scrolling shooters. Xevious is also the first to convincingly portray dithered/shaded organic landscapes as opposed to blocks-in-space or wireframe obstacles.

     Side-scrolling shoot 'em ups emerged in the early 1980s. Defender, introduced by Williams Electronics in late 1980 and entering production in early 1981, allowed side-scrolling in both directions in a wrap-around game world, unlike most later games in the genre. The scrolling helped remove design limitations associated with the screen, and it also featured a minimap radar. Scramble, released by Konami in early 1981, had continuous scrolling in a single direction and was the first side-scrolling shooter with multiple distinct levels.

     1985 saw the release of Konami's Gradius, which gave the player greater control over the choice of weaponry, thus introducing another element of strategy. The game also introduced the need for the player to memorise levels in order to achieve any measure of success. Gradius, with its iconic protagonist, defined the side-scrolling shoot 'em up and spawned a series spanning several sequels. The following year saw the emergence of one of Sega's forefront series with its game Fantasy Zone. The game received acclaim for its surreal graphics and setting and the protagonist, Opa-Opa, was for a time considered Sega's mascot. The game borrowed Defender's device of allowing the player to control the direction of flight and along with the earlier TwinBee (1985), is an early archetype of the "cute 'em up" subgenre. In 1986, Taito released KiKi KaiKai, an overhead multi-directional shooter. The game is notable for using a traditional fantasy setting in contrast to most shoot 'em up games filled with science fiction motifs. R-Type, an acclaimed side-scrolling shoot 'em up, was released in 1987 by Irem, employing slower paced scrolling than usual, with difficult, claustrophobic levels calling for methodicalstrategies. 1990's Raiden was the beginning of another acclaimed and enduring series to emerge from this period.