The technology of molecular genetics has profoundly altered the conduct of biomedical research. An entire universe of problems that in the past had been addressed only through conjecture, including whole genome analysis, can now be studied directly. The rapidity and scope of these changes in research capacity have in turn led to speculation that medicine will be radically altered by the application of genomics to everyday practice. To date, gene therapy and several new tests for genetic susceptibility have been successfully implemented, although their impact on medicine as a whole remains very limited and their future contribution is hotly contested. The potential of molecular genetics, like all technology, must be evaluated on the basis of established principles of clinical and epidemiologic research. The rhetoric of some enthusiasts focuses instead on an optimistic best-case scenario of the future of DNA science, and many of their most far-reaching claims cannot be substantiated on the basis of what is currently known. The tension between the long-term goal of public health and medicine to identify and remove the causes of ill health, in contrast to the development of technological innovations that can cure disease or identify individual susceptibility, emerges as a major theme in this debate.