Modern epidemiology has been criticized as being fragmented and reductionist, lacking coherent theories, and contributing little to the understanding of disease. These criticisms assume that epidemiology is a system of knowledge about health and disease, based on observation. In fact, consensus on the definition of the field is surprisingly elusive. A more practice-based approach would be to consider epidemiology as a field concerned with the methods for determining the causes of disease and for evaluating health services and treatments. The value of epidemiology is that it provides the tools for doing well-crafted research once the substantive hypothesis has been formulated. The focus of the discipline is thus methodological rather than conceptual. The major implication for the practitioner is that he or she must necessarily be more than an epidemiologist. Theoretical understanding must come from conceptual developments in the substantive fields of medicine, sociology, or biology and, more fundamentally, from fresh visions that transcend traditional categories of exposure and disease and accepted views of causation.