In 1098, Baldwin of Boulogne left the main Crusading army which was travelling south towards Antioch and Jerusalem, and went east to Edessa. There, he convinced its king, Thoros, to adopt him as a son and heir. Thoros was Greek Orthodox, and was disliked by his Armenian subjects; he was soon assassinated, although it is unknown if Baldwin had any part in this. In any case, Baldwin became the new ruler, taking the title of Count (as he had been called in Boulogne).
In 1100, Baldwin became king of Jerusalem when his brother Godfrey died. The County of Edessa passed to his cousin Baldwin of Bourcq, who was popular with the Armenians and took an Armenian wife. He was joined by Joscelin of Courtenay, who became lord of the fortress of Turbessel on the Euphrates, an important outpost against the Seljuk Turks.
Baldwin II quickly became involved in the affairs of northern Syria and Asia Minor. He helped secure the ransom of Bohemund I of Antioch from the Danishmends in 1103, and, with Antioch, attacked the Byzantine Empire in Cilicia in 1104. Later in 1104, Edessa was attacked by Mosul, and both Baldwin and Joscelin were taken prisoner when they were defeated at the fortress of Harran. Bohemund's brother Tancred became regent in Edessa, until Baldwin and Joscelin were ransomed in 1107. However, Baldwin had to fight to regain control of the city; Tancred was eventually defeated, though Baldwin had to ally with some of the local Muslim rulers.
In 1110, all lands east of the Euphrates were lost to Mawdud of Mosul; however, like the other attacks, this one was not followed by an assault on Edessa itself, as the Muslim rulers were more concerned with consolidating their own power.
Baldwin II became King of Jerusalem (also as Baldwin II) when Baldwin I died in 1118. Although Eustace of Boulogne had a better claim as Baldwin's brother, he was in France and did not want the title. Edessa was given to Joscelin in 1119. Joscelin was taken prisoner once again in 1122; when Baldwin came to rescue him, he too was captured, and Jerusalem was left without its king. However, Joscelin escaped in 1123, and obtained Baldwin's release the next year.
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; meanwhile, Joscelin II paid little attention to the security of his county, and argued with the counts of Tripoli who then refused to come to his aid. Zengi besieged the city in 1144, capturing it on December 24 of that year. Joscelin continued to rule in his lands west of the Euphrates, centered around Turbessel, until 1149, when he was captured in battle by Zengi's son Nur ad-Din; he was kept prisoner until he died in 1159. His wife sold what was left of the County to the Byzantine Emperor Manuel I Comnenus, and its remaining strong points were captured by Nur ad-Din and the Sultan of Rum within the next year. It was the first Crusader state to be captured, and also the first to be lost.
Edessa was one of the largest of the Crusader states in terms of territory. However, it was one of the smallest, by population. Edessa itself had about 10 000 inhabitants, but the rest of the county consisted mostly of fortresses. The county's territory extended from Antioch in the west to across the Euphrates in the west, at least at its greatest extent; it also often occupied land as far north as Armenia proper. To the south and east were the powerful Muslim cities of Aleppo and Mosul, and the Jazira (northern Iraq). The inhabitants mostly were Syrian, Jacobite, and Armenian Christians, with some Greek Orthodox and Muslims. Although the numbers of Latins always remained small, there was a Roman Catholic Patriarch, and the fall of the city was the catalyst for the Second Crusade in 1146.