The dual-use dilemma arises in the context of research in the biological and other sciences as a consequence of the fact that one and the same piece of scientific research sometimes has the potential to be used for bad as well as good purposes. It is an ethical dilemma since it is about promoting good in the context of the potential for also causing harm, e.g., the promotion of health in the context of providing the wherewithal for the killing of innocents. It is an ethical dilemma for the researcher because of the potential actions of others, e.g., malevolent nonresearchers who might steal dangerous biological agents, or make use of the original researcher's work. And it is a dilemma for governments concerned with the security of their citizens, as well as their health. In this article we construct a taxonomy of types of ''experiments of concern'' in the biological sciences, and thereby map the terrain of ethical risk. We then provide a series of analyses of the ethical problems and considerations at issue in the dual-use dilemma, including the impermissibility of certain kinds of research and possible restrictions on dissemination of research results given the risks to health and security. Finally, we explore the main availableinstitutional responses to some of the specific ethical problems posed by the dualuse dilemma in the biological sciences.