A lancer was a type of cavalryman who fought with a lance. Lances were used for mounted warfare in Assyria as early as 700 BC and subsequently by India, Egypt, China, Persia, Greece, and Rome. The weapon was widely used throughout Eurasia during the Middle Ages and the Renaissance by heavy cavalry, but fell out of general use by the late 16th century, before its revival by light cavalry in the early 19th century. Lance cavalry remained in an active role into the early 20th century and World War I. In modern times, many militaries retain units designated as lancers. However, the lance itself has been relegated to a ceremonial role.

The lancer (Polish: ułan, German: Ulan, French: uhlan) had become a common sight in the majority of European, Ottoman, and Indian cavalry forces during this time, but, with the exception of the Ottoman troops, they increasingly discarded the heavy armour to give greater freedom of movement in combat. The Polish "winged" lancers (hussars) were amongst the last European units to abandon their armour. There was debate over the value of the lance in mounted combat during the 17th and 18th centuries, with most armies having very few lancer units by the beginning of the 19th century.

However, during the Napoleonic Wars, lancers were to be seen in many of the combatant nations as their value in shock tactics became clear. During the wars, the Poles became a ready source of recruitment for several armies, willingly or unwillingly. Polish lancers served with distinction in the Austrian, Prussian, Russian, and French armies, most famously in Napoleon's French Imperial Guard as the 1er Regiment de Chevau-Legers-Lanciers de la Garde Impériale.

At the Battle of Waterloo, French lances were "nearly 3 metres (9.8 ft) long, weighed around 3 kilograms (6.6 lb), and had a steel point on a wooden staff," according to historian Alessandro Barbero. He adds that they were "terrifyingly efficient". Commander of the French 1st Corps, 4th Division General Durutte, who saw the battle from the high ground in front of Papelotte, would write later, "I had never before realized the great superiority of the lance over the sword."

Although having substantial impact in the charge, lancers could be more vulnerable to other cavalry units in close quarters combat, where the lance proved to be a clumsy and easily deflected weapon when employed against sabres in a mêlée. By the late 19th century, many cavalry regiments in Eurasian armies were composed of troopers with lances in the front rank and those with sabres in the second: the lances for the initial shock and sabres for the ensuing mêlée.

Lancers typically wore a double-breasted jacket (kurtka) with a coloured panel at the front (plastron), a coloured band of cloth (sash), and a square-topped cap (czapka). Their lance usually had a small swallow-tailed flag (pennon), just below the lance head. The pennons were normally removed or wrapped in a canvas cover during active service ^{[citation needed]}. With the improved range and accuracy of infantry muskets and rifles, the high profile presented by lancers with their conspicuous weapons became a problem. Lancers were trained to lower their lances when scouting on hilltops, to help avoid detection by enemy combatants ^{[citation needed]}.

In 1914, lances were still being carried by regiments in the British, Indian, French, Prussian, Italian, Chilean, Portuguese, Japanese, Spanish, Ottoman, Belgian, Argentine, Russian, and Siamese armies, among others. Almost all German cavalry branches (cuirassiers, hussars, dragoons, and uhlans) retained steel tube lances (stahlrohrlanze) 3.2 meters (10 ft) in length, as their primary weapon. As late as 1914, half of the troopers in each Russian regular cavalry regiment (hussars, uhlans, and dragoons) carried lances on active service, as did all cossacks.

The British cavalry lost the lance for all but ceremonial use in 1903, following the Second Boer War. However, a conservative revaluation led to its reintroduction as an active service weapon from 1909 to 1928.