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 is distinct from heavy infantry. Heavy infantry are dedicated primarily to fighting in tight formations that were the core of large battles. Light infantry often fought in close coordination 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.

The concept of a skirmishing screen is a very old one and was already well-established by Greek and Roman times. 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 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. Unusually, their officers often carried muskets as well and their swords were lighter and curved, as opposed to the heavy, straighter swords of other infantry officers. Orders were sent by bugle 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 heavy infantry. By the late 19th century the concept of fighting in formation was on the wane and the distinctions between light and heavy infantry began to disappear. Essentially, all infantry became light infantry in practice. Some regiments retained the name and customs, but there was in effect no difference between them and other infantry regiments.

Today, the term light infantry is sometimes used to refer to infantry who are not transported in armoured fighting vehicles, instead walking or using unarmoured vehicles.

The Light Infantry is also the name of a regiment in the British Army formed by the amalgamation of four of the old light infantry regiments.