Demographic theory is concerned with how population systems regulate themselves given available resources and external shocks to population size. This chapter provides an overview of demographic theory, focusing specifically on relationships between population size, population growth, and welfare. It then discusses four implications of demographic theory for population ethics. Speaking broadly, these four implications concern (1) the overreliance by some population ethicists on Malthusian assumptions about the average welfare of population declining with increasing population size, (2) the likelihood of certain hypothetical scenarios that feature in thought experiments used in population ethics, (3) the prioritization of extinction risks by population ethicists, and (4) the patterns of intergenerational and intertemporal inequality that population ethicists may anticipate over the long run. The chapter closes with a discussion of demographic theory in relation to historical and future demographic change.