Skirmishers are light infantry or light cavalry soldiers deployed as a vanguard, flank guard or rearguard to screen a tactical position or a larger body of friendly troops from enemy advances. They are usually deployed in a skirmish line, an irregular open formation that is much more spread out in depth and in breadth than a traditional line formation. Their purpose is to harass the enemy by engaging them in only light or sporadic combat to delay their movement, disrupt their attack, or weaken their morale. Such tactics are collectively called skirmishing.

A battle with only light, relatively indecisive combat is often called a skirmish even if heavier troops are sometimes involved.

Skirmishers can be either regular army units that are temporarily detached to perform skirmishing or specialty units that are specifically armed and trained for such low-level irregular warfare tactics. Light infantry, light cavalry, and irregular units often specialize in skirmishing. Skirmishers' open formations and smaller numbers can give them superior mobility over the regular forces, allowing them to engage only on favorable terms, taking advantage of better position or terrain, and quickly withdrawing from any threat of superior enemy forces.

Though often critical in protecting the main army from sudden enemy advances, skirmishers are poor at taking or defending ground from heavy infantry or heavy cavalry. In modern times, following the obsolescence of such heavy troops, all infantry has become indistinguishable from skirmishers, and the term has effectively lost its original military meaning as a distinct class of soldier, although skirmishing as a combat role is commonplace.