Laser tag is a team or individual sport or recreational activity where players attempt to score points by tagging targets, typically with a hand-held infrared-emitting targeting device. Infrared-sensitive targets are commonly worn by each player and are sometimes integrated within the arena in which the game is played. Since its birth in 1984, with the release of the Star Trek Electronic Phasers toy manufactured by the South Bend Electronics brand of Milton Bradley, laser tag has evolved into both indoor and outdoor styles of play, and may include simulations of combat, role play-style games, or competitive sporting events including tactical configurations and precise game goals.

Laser tag is popular with a wide range of ages. When compared to paintball, laser tag is painless because it uses no physical projectiles, and indoor versions may be considered less physically demanding because most indoor venues prohibit running or roughhousing.

In late 1970s and early 1980s, the United States Army deployed a system using infrared beams for combat training. The MILES system functions like laser tag in that beams are "fired" into receivers that score hits. Similar systems are now manufactured by several companies and used by various armed forces around the world.

A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation. The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies.

Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Laser beams can be focused to very tiny spots, achieving a very high irradiance. Or they can be launched into a beam of very low divergence in order to concentrate their power at a large distance.

Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase which vary randomly with respect to time and position, and thus a very short coherence length.

Tag or tagging may refer to: