Light infantry refers to certain types of lightly-equipped infantry throughout history. They have a more mobile or fluid function than other types of infantry, such as heavy infantry or line infantry. Historically, light infantry often fought as scouts, raiders, and skirmishers. These are loose formations that fight ahead of the main army to harass, delay, disrupt supply lines, engage the enemy's own skirmishing forces, and generally "soften up" an enemy before the main battle. Light infantrymen were also often responsible for screening the main body of a military formation.
Post-World War II, the term "light infantry" evolved to include rapid-deployment units (including commandos and airborne units) that emphasize speed and mobility over armor and firepower. Some units or battalions that historically held a skirmishing role have kept their designation "light infantry" for the sake of tradition.
The concept of a skirmishing screen is a very old one and was already well-established in Ancient Greece and Roman times in the form, for example, of the Greek peltast and psiloi, and the Roman velites. As with the 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 in a phalanx formation that made them heavy infantry.
Early regular armies of the modern era frequently relied on irregulars to perform the duties of light infantry skirmishers. In particular, the French Army employed detachments of German and Balkan mercenaries to serve as patrols in the rough country until a permanent corps of Mountain Fusiliers (Fusiliers des Montagnes) was raised in the 1740s. In the 17th century, dragoons were sometimes employed as the skirmishers of their day – mounted infantrymen who rode into battle but dismounted to fight, giving them a mobility lacking to regular foot soldiers.