The Church of the East, also known as the Nestorian Church,is a Christian church, part of the Syriac tradition of Eastern Christianity.

Originally the church of the Persian Sassanid Empire, it quickly spreads widely through Asia.

Between the 9th and 14th centuries it is the world's largest Christian church in terms of geographical extent, with dioceses stretching from the Mediterranean to China and India.

Several modern churches claim continuity with the historical Church of the East.The Church of the East is headed by the Patriarch of the East, continuing a line that, according to tradition, stretches back to the time of the apostles.

Liturgically, the church adheres to the East Syrian Rite, and theologically, it is associated with the doctrine of Nestorianism, which emphasizes the distinctness of the divine and human natures of Jesus.

This doctrine and its chief proponent, Nestorius (386–451), are condemned by the First Council of Ephesus in 431, leading to the Nestorian Schism and a subsequent exodus of Nestorius' supporters to Sassanid Persia.

The existing Christians in Persia welcome these refugees and gradually adopt Nestorian doctrine, leading the Church of Persia to be known alternately as the Nestorian Church.The church grows rapidly under the Sassanids, and following the Islamic conquest of Persia, it is designated as a protected dhimmi community under Muslim rule.

From the 6th century, it expandes greatly, establishing communities in India (the Saint Thomas Christians), Central Asia (where they havc evangelical success among the Mongol tribes), and China, which is home to a thriving Nestorian community under the Tang Dynasty from the 7th to the 9th century.

In the 13th and 14th centuries, the church experiences a final period of expansion under the Mongol Empire, which has influential Nestorian Christians in the Mongol court.From its peak of geographical extent, the church experiences a rapid period of decline starting in the 14th century, due in large part to outside influences.

The Mongol Empire dissolves into civil war, the Chinese Ming Dynasty overthrows the Mongols and ejects Christians and other foreign influences from China (also including Manichaeism), and many Mongols in Central Asia convert to Islam.

The Muslim Mongol leader Timur (1336–1405) nearly eradicates the remaining Christians in Persia; hereafter, Nestorian Christianity is largely confined to Upper Mesopotamia and the Malabar Coast of India.

In the 16th century, the Church of the East goes into a schism from which two distinct churches eventually emerge; the modern Assyrian Church of the East and the Chaldean Catholic Church, an Eastern Catholic Church in communion with the Holy See.