The Crusader states, or Outremer, were four Catholic polities that existed in the Levant from 1098 to 1291. Following the principles of feudalism, the foundation for these polities was laid by the First Crusade, which was proclaimed by the Latin Church in 1095 in order to reclaim the Holy Land after it was lost to the 7th-century Muslim conquest. Situated on the Eastern Mediterranean, the four states were, in order from north to south: the County of Edessa (1098–1150), the Principality of Antioch (1098–1268), the County of Tripoli (1102–1289), and the Kingdom of Jerusalem (1099–1291). The three northern states covered an area in what is now southeastern Turkey, northwestern Syria, and northern Lebanon; and the Kingdom of Jerusalem, the southernmost and most prominent state, covered an area in what is now Israel, Palestine, southern Lebanon, and western Jordan. The description "Crusader states" can be misleading, as from 1130 onwards, very few people among the Franks were Crusaders. Medieval and modern writers use the term "Outremer" as a synonym, derived from the French word for overseas.

The First Crusade (1096–1099) was the first of a series of religious wars, or Crusades, initiated, supported and at times directed by the Latin Church in the middle ages. The objective was the recovery of the Holy Land from Islamic rule. While Jerusalem had been under Muslim rule for hundreds of years, by the 11th century the Seljuk takeover of the region threatened local Christian populations, pilgrimages from the West, and the Byzantine Empire itself. The earliest initiative for the First Crusade began in 1095 when Byzantine emperor Alexios I Komnenos requested military support from the Council of Piacenza in the empire's conflict with the Seljuk-led Turks. This was followed later in the year by the Council of Clermont, during which Pope Urban II supported the Byzantine request for military assistance and also urged faithful Christians to undertake an armed pilgrimage to Jerusalem.