The Hundred Years' War (French: Guerre de Cent Ans) was a series of separate wars lasting from 1337 to 1453 between two royal houses for the French throne, which was vacant with the extinction of the senior Capetian line of French kings. The two primary contenders were the House of Valois and the House of Plantagenet, also known as the House of Anjou. The House of Valois claimed the title of King of France, while the Plantagenets from England claimed to be Kings of France and England. Plantagenet Kings were the 12th century rulers of the Kingdom of England, and had their roots in the French regions of Anjou and Normandy. French soldiers fought on both sides, with Burgundy and Aquitaine providing notable support for the Plantagenet side.

The conflict lasted 116 years but was punctuated by several periods of peace, before it finally ended in the expulsion of the Plantagenets from France (except the Pale of Calais). The war was eventually a victory for the house of Valois, who succeeded in recovering the Plantagenet gains made initially and expelling them from the majority of France by the 1450s. However, the war nearly ruined the Valois, while the Plantagenets gained huge amounts of plunder from the mainland, which enriched England. France itself likewise suffered greatly from the war, as most of the conflict occurred on the continent.

The war was in fact a series of conflicts and is commonly divided into three or four phases: the Edwardian War (1337–1360), the Caroline War (1369–1389), the Lancastrian War (1415–1429), and the slow decline of English fortunes after the appearance of Joan of Arc (1412–1431). Several other contemporary European conflicts were directly related to this conflict: the Breton War of Succession, the Castilian Civil War, the War of the Two Peters, and the 1383-1385 Crisis. The term "Hundred Years' War" was a later term invented by historians to describe the series of events.

The war owes its historical significance to a number of factors. Though primarily a dynastic conflict, the war gave impetus to ideas of both French and English nationalism. Militarily, it saw the introduction of new weapons and tactics, which eroded the older system of feudal armies dominated by heavy cavalry. The first standing armies in Western Europe since the time of the Western Roman Empire were introduced for the war, thus changing the role of the peasantry. For all this, as well as for its long duration, it is often viewed as one of the most significant conflicts in the history of medieval warfare. In France, the English invasion, civil wars, deadly epidemics, famines and marauding mercenary armies (turned to banditry) reduced the population by two-thirds. Shorn of its Continental possessions, England was left an island nation, a fact which profoundly affected its outlook and development for more than 500 years.