In Christian theology, ecclesiology is the study of the Church, the origins of Christianity, its relationship to Jesus, its role in salvation, its polity, its discipline, its eschatology, and its leadership.

In its early history, one of the Church's primary ecclesiological issues had to do with the status of Gentile members in what had become the New Testament fulfilment of the essentially Jewish Old Testament church. It later contended with such questions as whether it was to be governed by a council of presbyters or a single bishop, how much authority the bishop of Rome had over other major bishops, the role of the Church in the world, whether salvation was possible outside of the institution of the Church, the relationship between the Church and the State, and questions of theology and liturgy and other issues. Ecclesiology may be used in the specific sense of a particular church or denomination's character, self-described or otherwise. This is the sense of the word in such phrases as Catholic ecclesiology, Protestant ecclesiology, and ecumenical ecclesiology.

The word ecclesiology was defined in the 19th century as the science of the building and decoration of church buildings and is still used in that sense in the context of architectural history.