The Assyrian Church of the East (ACOE), sometimes called the Church of the East and officially known as the Holy Apostolic Catholic Assyrian Church of the East (HACACE), is an Eastern Christian church that follows the traditional Christology and ecclesiology of the historical Church of the East. It belongs to the eastern branch of Syriac Christianity, and employs the Divine Liturgy of Saints Addai and Mari belonging to the East Syriac Rite. Its main liturgical language is Classical Syriac, a dialect of Eastern Aramaic, and the majority of its adherents are ethnic Assyrians.

Eastern Christianity comprises Christian traditions and church families that originally developed during classical and late antiquity in the Eastern Mediterranean region or locations further east, south or north. The term does not describe a single communion or religious denomination. Eastern Christianity is a category distinguished from Western Christianity, which is composed of those Christian traditions and churches that originally developed further west.