Seat of a prosperous felt trade, Edessa was seized by Baldwin of Bologne in 1098CE through a rather ususal method. A western Crusader, he had left the army of the First Crusade and somehow convinced Thoros of Edessa, lord of the city, to adopt him as heir. This was done through a curious ceremony where he and Baldwin rubbed their bare chests together. Thoros, a Greek Orthodox Christian, was unpopular with his Armenian Orthodox subjects and shortly after the adoption Thoros was killed in religious rioting, which Baldwin quickly quelled, becoming the undisputed Count of Edessa.
In 1100, Baldwin inherited the throne of the new Kingdom of Jerusalem when his brother Godfrey of Bouillon died. The County of Edessa passed to his cousin Baldwin of Bourcq. He was joined by Joscelin of Courtenay, who took the fortress of Turbessel on the Euphrates as his fief; it quickly became an important outpost against the Seljuk Turks.
Baldwin of Bourcq immediately engaged in a series of wars against his Muslim neighbours, during which his fortunes waxed and waned. In 1110, for example, he lost all his lands east of the Euphrates. Upon the death of Baldwin I in 1118 Baldwin of Bourcq would himself inherit the throne of Jerusalem and ceded the county to Joscelin. A break in the rulership occurred in 1122, when Joscelin was taken prisoner by the Turks; Baldwin attempted to rescue him and was likewise imprisoned for more than a year. By 1124, however, both the Count of Edessa and the King of Jerusalem had been ransomed and they resumed their respective seats.
Joscelin was killed in battle in 1131 and was succeeded by his son Joscelin II. By this time, however, Zengi had united Aleppo and Mosul and began to threaten Edessa. Joscelin II paid little attention to the security of his county, instead quarreling with the Counts of Tripoli who would later refuse to come to his aid.
Zengi besieged the city in 1144, capturing it on December 24. Joscelin continued to rule his lands west of the Euphrates, and he also managed to take advantage of the death of Zengi in September 1146 to regain and hold (albeit briefly) his old capital. The city was again lost in November, and Joscelin barely escaped. In 1150 he was captured by Zengi's son Nur al-Din, and was kept a prisoner in Aleppo until he died in 1159. The title of Count of Edessa was in use by Joscelin's descendants during the 12th century, but fell into disuse when it became clear that the county would remain lost.
The fall of Edessa was the catalyst for the Second Crusade in 1146, although the crusade never attempted to liberate the city; it remained a Turkish possession into the modern era.