Scientific and ethical theories are examined in terms of the practice of epidemiology, the study of the determinants and distributions of disease, and the application of the knowledge gained to prevent disease and improve the health of populations. Scientific theories provide explanations and predictions for epidemiologic studies of disease etiology and prevention. Causal theory is the key example for epidemiologic science, although theories of selection processes, theories of health, and theories of probability and statistics are also core. Theories of ethics provide principles, warrants, and methods for acting upon the knowledge gained in the scientific pursuits of epidemiology and for obtaining that knowledge appropriately. Ethical theories guide practitioners in making justified decisions about when and under what conditions public health interventions should be undertaken and how research participants should be treated. Theories of midlevel bioethical principles have received the most attention in epidemiology, but other theories, such as virtue theory and communitarian theory, are also relevant. Many theories matter to the practice of epidemiology: theories of biology, aging, evolution, and medicine; theories of history, religion, law, economics, and politics, as well as the theories of the physical, behavioral, and social sciences. These theories matter because epidemiologists study many different biological and social phenomena, and preventive interventions occur at many different levels of explanation. Development of theory is not a high priority in contemporary epidemiology. Identifying the responsibilities of the discipline to be public health intervention and rigorous science is a first step toward developing and applying theory in epidemiology.