This chapter examines the formation and development of Unitarian liturgy and worship practices from the sixteenth to the nineteenth centuries. Despite the absence of a sixteenth- and seventeenth-century order of service, a hypothetical reconstruction of the Unitarian order of service is attempted using the earliest Unitarian hymnal. Using reliable written sources such as published prayer and hymn books and early ecclesiastical regulations, supplemented by synodical resolutions, the challenges of reconstructing a unified Unitarian liturgy are presented.