This encyclopedia entry traces the life of Nestorius (c. 385-451), onetime archbishop of Constantinople, heresiarch and putative founder of 'Nestorian' church. We examine his life, the Nestorian controversy in context. Special consideration is given to the question of whether the heterodoxy of Nestorius' position was crucial for the church's later constitution. A brief history of the Nestorian, or East Syrian church over its vast territory is also appended from the late 4th century to the present.