This chapter will consider the ritual structure and elements of the Roman Mass in the crucial period of formation from the fifth to the early eighth century, which is especially associated with the decisive contributions of popes, such as Leo the Great (r. 440–461), Gelasius (r. 492–496) and Gregory the Great (r. 590–604). The reader will be introduced to the liturgical books that were compiled for the celebration of the Mass (sacramentaries, lectionaries, antiphoners, ordines). Particular attention will then be given to the papal stational liturgy as described in Ordo Romanus I, which was to have a fundamental impact on later development in the West. The chapter will include an extended discussion of the role of chant in the Mass, as well as sections on the renewed presence of Greek in the Roman liturgy and on the direction of liturgical prayer.