The Great Seljuk Empire was the Turkish state which dominated the Middle East and Central Asia in the eleventh and twelfth centuries. This book surveys that period, which was one of exceptional importance, witnessing profound demographic, religious, political and social changes in the Islamic Middle East. The Turkish invasions played a role in provoking the Crusades, led to the collapse of Byzantine power in Anatolia and brought about the beginnings of Turkish settlement in what is now Turkey and Iran, permanently altering their ethnic and linguistic composition.