Traditionally light infantry (or skirmishers) were soldiers whose job was to provide a skirmishing screen ahead of the main body of infantry, harassing and delaying the enemy advance. Light infantry was distinct from medium, heavy or line infantry. Heavy infantry were dedicated primarily to fighting in tight formations that were the core of large battles. Light infantry often fought in close co-ordination with heavy infantry, where they could screen the heavy infantry from harassing fire, and the heavy infantry could intervene to protect the light infantry from attacks of enemy heavy infantry or cavalry. Heavy infantry originally had heavier arms and more armour than light infantry, but this distinction was lost as the use of armour declined and gunpowder weapons became standardized.

Antiquity

The concept of a skirmishing screen is a very old one and was already well-established by Greek and Roman times in the form, for example, of the Greek peltast and the Roman velites. As with so called 'light infantry' of later periods, the term more adequately describes the role of such infantry rather than the actual weight of their equipment. Peltast equipment for example grew steadily heavier at the same time as hoplite equipment grew lighter. It was the fact that peltasts fought in open order as skirmishers that made them light infantry, and that hoplites fought in the battle line as a phalanx that made them heavy infantry.

Modern age

Regular armies usually relied on irregulars to perform the duties of light infantry skirmishers. Later, the dragoons of the 17th century were the light infantry skirmishers of their day  lightly-armed and armoured infantrymen who rode into battle but dismounted to fight.

In the 18th and 19th centuries most infantry battalions had a light company. Its members were usually smaller, agile men capable of using their initiative, since they did not always fight in disciplined ranks as did the ordinary infantry but often in widely dispersed groups. They were also often chosen for their shooting ability and sometimes carried lighter muskets than ordinary infantrymen. Some light infantry units carried rifles instead of muskets, and wore rifle green uniforms; they became designated as rifle regiments in Britain and Jäger regiments in German speaking Europe. In France, during the Napoleonic Wars, light infantry were called voltigeurs and the sharpshooters tirailleurs.

Unusually, light infantry officers sometimes carried muskets as well and their swords were lighter and curved sabres; as opposed to the heavy, straighter swords of other infantry officers. Orders were sent by bugle or whistle instead of drum (since the sound of a bugle carries further and it is difficult to move fast when carrying a drum). Some armies, including the British and French, converted whole regiments into light infantry. These were sometimes considered elite units, since they required more training and self-discipline to carry out the roles of light infantry as well as those of ordinary infantry.