The County of Edessa (Latin: Comitatus Edessanus) was one of the Crusader states in the 12th century. Its seat was the city of Edessa (present-day anlurfa, Turkey).

In the late Byzantine period, Edessa became the centre of intellectual life within the Syriac Orthodox Church. As such it also became the centre for the translation of Ancient Greek philosophy into Syriac, which provided a stepping stone for the subsequent translations into Arabic. When the Crusades arrived, it was still important enough to tempt a side-expedition after the siege of Antioch.

Baldwin of Boulogne, the first Count of Edessa, became King of Jerusalem, and subsequent counts were his cousins. Unlike the other Crusader states, the County was landlocked. It was remote from the other states and was not on particularly good terms with its closest neighbor, the Principality of Antioch. Half of the county, including its capital, was located east of the Euphrates, far to the east, rendering it particularly vulnerable. The west part of the Euphrates was controlled from the stronghold of Turbessel. The eastern border of Edessa was the Tigris, but the County may not have extended quite that far.

The fall of Edessa in 1144 was the first major setback for Outremer and provoked the Second Crusade. All the later Crusades, however, were troubled by strategic uncertainties and disagreements. The Second Crusade did not even try to recover Edessa, calculating it to be strategically better to take Damascus. But the campaign failed and Edessa was lost for the Christians.

The Crusades were a series of religious wars initiated, supported, and sometimes directed by the Latin Church in the medieval period. The best known of these Crusades are those to the Holy Land in the period between 1095 and 1291 that were intended to recover Jerusalem and its surrounding area from Islamic rule. Concurrent military activities in the Iberian Peninsula against the Moors (the Reconquista) and in northern Europe against pagan West Slavic, Baltic and Finnic peoples (the Northern Crusades) also became known as crusades. Through the 15th century, other church-sanctioned crusades were fought against heretical Christian sects, against the Byzantine and Ottoman empires, to combat paganism and heresy, and for political reasons. Unsanctioned by the church, Popular Crusades of ordinary citizens were also frequent. Beginning with the First Crusade which resulted in the recovery of Jerusalem in 1099, dozens of Crusades were fought, providing a focal point of European history for centuries.

In 1095, Pope Urban II proclaimed the First Crusade at the Council of Clermont. He encouraged military support for Byzantine emperor Alexios I against the Seljuk Turks and called for an armed pilgrimage to Jerusalem. Across all social strata in western Europe, there was an enthusiastic popular response. The first Crusaders had a variety of motivations, including religious salvation, satisfying feudal obligations, opportunities for renown, and economic or political advantage. Later crusades were generally conducted by more organized armies, sometimes led by a king. All were granted papal indulgences. Initial successes established four Crusader states: the County of Edessa; the Principality of Antioch; the Kingdom of Jerusalem; and the County of Tripoli. The Crusader presence remained in the region in some form until the fall of Acre in 1291. After this, there were no further crusades to recover the Holy Land.

Proclaimed a crusade in 1123, the struggle between the Christians and Muslims in the Iberian Peninsula was called the Reconquista by Christians, and only ended in 1492 with the fall of the Muslim Emirate of Granada. From 1147, campaigns in Northern Europe against pagan tribes were considered crusades. In 1199, Pope Innocent III began the practice of proclaiming political crusades against Christian heretics. In the 13th century, crusading was used against the Cathars in Languedoc and against Bosnia; this practice continued against the Waldensians in Savoy and the Hussites in Bohemia in the 15th century and against Protestants in the 16th. From the mid-14th century, crusading rhetoric was used in response to the rise of the Ottoman Empire, only ending in 1699 with the War of the Holy League.