The Opsician Theme or simply Opsikion was a Byzantine theme located in northwestern Asia Minor. Created from the imperial retinue army, the Opsikion was the largest and most prestigious of the early themes, being located closest to Constantinople. Involved in several revolts in the 8th century, it was split in three after c. 750, and lost its former pre-eminence. It survived as a middle-tier theme until after the Fourth Crusade.