What does it mean to be the same organism over time? This chapter develops an understanding of diachronic identity of organisms from an organizational perspective. We argue that a necessary condition for diachronic identity is organizational continuity, i.e., the presence of a continuous causal process linking successive organizational regimes, irrespective of material and functional changes. Organizational 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 organized systems, which occurs through fission, fusion, or a combination of the two. We discuss the utility of the resulting organizational view, as well as its relations with other approaches to biological individuality.