One of the basic questions in spatial ecology is: how much space does a population need to persist? The critical patch-size is the size of the suitable habitat where population gain through reproduction balances population loss through dispersal. The question of how large a certain habitat has to be to support a given population has important applications in conservation biology, e.g., when designing a protected area to ensure the survival of an endangered population. The analysis in this chapter is based on linearization, thereby implicitly assuming that the population growth function has no Allee effect. We explicitly compute the critical size when dispersal is described by a Laplace kernel. We then compare how different dispersal patterns affect this critical size. At the end of the chapter, we consider the class of separable kernels and introduce an approximation method.