The survival models considered earlier in the book conditioned on the time of the first capture and where necessary also the state in which an individual was captured. See for example Sections 4.3 and 5.1. Previously, in Chapter 3, we have considered abundance estimation only for closed populations, with no mortality, and as a result models for abundance and survival have been easily separated. Here we consider more complex models such as those for estimating population size for open populations in which births and deaths can occur within the study period, and models which include open and closed periods within a single study will also be presented.