A population's age structure and growth are determined by rates of fertility, mortality and migration. Stable population theory provides a widely useful mathematical framework, described here, that connects a fixed set of rates to the population dynamics they generate. This theory makes it possible to trace causes and consequences of population change, to establish methods for estimating rates, and to make projections of future population. Much of the power of stable theory rests on the fact that the key features of population dynamics with fixed rates can be generalized, as discussed here, to rates that vary over time.