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.
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.
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.
In the 1980s, the United States Army increased light forces to address contingencies and increased threats requiring a more deployable force able to operate for in restrictive environments for limited periods. At its height, this included the 10th Mountain Division (Light Infantry), 7th Infantry Division (Light), 25th Infantry Division, 6th Infantry Division (Light), 82nd Airborne Division, 101st Airborne Division (AASLT), 75th Ranger Regiment, and other battalion and brigade combat teams in mixed heavy/light units. Operation Just Cause is often cited as proof of concept. Almost 30,000 US Forces, mostly light, deployed to Panama within a 48 hour period to execute combat operations.
During the 1990s the concept of purely light forces came under scrutiny due to their decreased lethality and survivability. This scrutiny has resulted in Stryker Brigade Combat Teams, a greater focus on task organized units (such as Marine Expeditionary Units) and a reduction of purely light forces.
Examples of current light units:
Note that in some armies Light Infantry are usually considered as an elite, but in other countries they may be considered inferior due to their lack of equipment.