Epidemiology and demography, although both encumbering several theoretical constructs, are generally applied, problem-solving disciplines concerned with the nature and trajectories of defined groups. There are differences between the two disciplines in substantive interests and methodological traditions, however both have comprehensive perspectives and both are deeply concerned with the health, social, and economic well-being of human populations. One way to promote productive interdisciplinary research is to apply or develop scientific theories that exploit the complementary interests and methods of the two disciplines. Several candidate theories and themes are suggested here, including (1) the life course approach as applied to the biology of longevity; (2) the "modernized" Malthusian dilemma; (3) the demographic transition within developing countries; (4) the theory of evolution and its social and biological implications; (5) reciprocal effects of ecological and environmental characteristics on population health and well-being; and (6) systems theory as an approach to population complexity. Other approaches to fostering interdisciplinary investigation between these disciplines include joint course work, specifically targeted research funding initiatives and collaborative development of new population theory. The latter may be particularly important from several perspectives: uniting disciplines to jointly approach common and important scientific problems, providing existing frameworks with which to generate new scientific questions, and promoting enhanced scientific rigor by more tightly linking methods with hypotheses.