The birth and evolution of a scientific method is an exciting development. Meta-analysis, described in this chapter as “one of the most important and controversial methodological developments in the history of science,” has changed aspects of scientific inquiry in ways that have not been fully calculated. The technique is nearly as old as this century but as fresh, immediate, and important as this week's journal articles and subsequent lay accounts. In a meta-analysis, the results of previous studies are pooled and then analyzed by any of a number of statistical tools. Meta-analyses are performed on data stored in computers and subjected to computational statistics. The technique grew rapidly in psychology beginning two decades ago and since has become a fixture in the observational investigations of epidemiologists, in reviews of clinical trials in medicine, and in the other health sciences. It has engendered extraordinarily heated debate about its quality, accuracy, and appropriate applications. Meta-analysis raises ethical issues because doubts about its accuracy raise doubts about (1) the proper protections of human subjects in clinical trials; (2) the proper treatment of individual patients by their physicians, nurses, and psychologists; and (3) the correct influence on public policy debates. This chapter lays out ethical and policy issues, and argues for high educational standards for practitioners in each domain.