This paper examines the relationships between natural increase (births minus deaths) and net migration (in-migrants minus out-migrants) in regional populations and identifies and clarifies some of the fundamental population dynamics involved. First the authors consider several well-defined regularities of fertility mortality and migration schedules. Next they study some of the principal population dynamincs that connect such schedules with the growth age composition and spatial distribution of multiregional populations that are subjected to them. Finally they examine some of the spatial implications of population growth. Knowledgeable users of population projections know that the assumptions that generated them are certain to be violated. This is especially true of multiregional projections that assume fixed schedules of internal migration. Unlike fertility or mortality migration is functionally related to 2 populations instead of 1. Therefore the assumption that age-specific migration rates between 2 regional populations will remain unaffected by changes in population size over time is unrealistic. Linear multiregional models of population growth can provide a better understanding of the spatial dynamics of growth than nonlinear models.