Constantine X Doukas or Ducas (; – 23 May 1067), was Byzantine emperor from 1059 to 1067. He was the founder of the Doukid dynasty. During his reign, the Normans took over much of the remaining Byzantine territories in Italy, while in the Balkans the Hungarians occupied Belgrade. He also suffered defeats by the Seljuk sultan Alp Arslan.
Reign
left|thumb|160px|A gold [[tetarteron of Constantine X]]
Constantine's parents are not mentioned in any primary sources, but some scholars theorize that he was the son of Andronikos Doukas, a nobleman who served as strategos of Preslav during the reign of Basil II (). Historians often give his birthdate as 1006, as he is said to have died aged "slightly over sixty years". He was an academic, addicted to endless debates about philosophy and theology, and he gained influence after he married, as his second wife, Eudokia Makrembolitissa, a niece of Patriarch Michael Keroularios. In 1057, Constantine supported the usurpation of Isaac I Komnenos, gradually siding with the court bureaucracy against the new emperor's reforms.
His decisions to replace standing soldiers with mercenaries and leave the frontier fortifications unrepaired led Constantine to become naturally unpopular with the supporters of Isaac within the military aristocracy, who attempted to assassinate him in 1061.