This chapter starts with Augustine because his work sets the agenda for the mysticism of the high and late Middle Ages. The chapter also provides reference to this earlier work, which provides the background against which monks, nuns, members of the religious movements and eventually the laity lived, practised, and thought about the mystical life. More accurately, perhaps, his descriptions, in the Confessions and elsewhere, of the transcendence, transience, ineffability, and communal nature of mystical experience provide the parameters within which medieval Christians understand experiences of God and of union with God. Medieval Christian mysticism can be understood as a series of ongoing experiential, communal, and textual commentaries on and debates about the possibilities and limitations Augustine sets for the earthly encounter between God and humanity. Medieval Christian mysticism can be understood as a series of ongoing experiential, communal, and textual commentaries on and debates about the possibilities and limitations Augustine sets for the earthly encounter between God and humanity.