The Chronicle of Montecassino, begun by Leo Marsicanus at the end of the eleventh century, is a huge and complex work. After outlining how and why it was begun, and the structure of the work, this study turns to the continuation of Leo’s chronicle, covering the years 1072-1138. It discusses the authorship of the continuation, and in particular the role of the final continuator Peter the Deacon, before turning to the themes and concerns of the various authors, and what these may tell us of the monastery of Montecassino during the first half of the twelfth century, at a time when the monks felt that their prestige, material interests and independence were increasingly under threat.