What does it mean to be the same organism over time? This chapter develops an understanding of diachronic identity of organisms from an organisational perspective. We argue that a necessary condition for diachronic identity is organisational continuity, i.e., the presence of a continuous causal process linking successive organisational regimes, irrespective of material and functional changes. Organisational continuity is not a sufficient condition, however, because it cannot discriminate between the development of the same individual and the reproduction of a new individual. We therefore suggest that there are temporal boundaries of identity when there are changes in the number of continuous organised systems, which can occur through fission, fusion, or a combination of the two. We discuss the utility of the resulting organisational view, as well as its relations with other approaches to biological individuality.