Statements of ethical norms can be found in the various codes governing research with human subjects. Most of the codes and guidelines embrace similar norms-there should be good research design, competent investigators, a favorable balance of potential harms and anticipated benefits, voluntary and informed consent, and equitable selection of subjects. The codes have evolved over time. The first international code, the Nuremberg Code, contained a set of 10 principles for ethical human experimentation and was written in 1947 by the U.S. Military Tribunal that prosecuted high-ranking Nazi physicians following World War II. The Nuremberg Code made the voluntary consent of the subject “absolutely essential.” But investigators found this unworkable because a strict reading of it would forbid research involving children, the dying, the mentally ill, and other populations incapable of exercising consent. In 1964 the World Medical Association promulgated a set of guidelines, the purpose of which was to interpret the principles of the Nuremberg Code so as to make them relevant and responsive to the empirical facts of medical research. These guidelines, known as the Declaration of Helsinki, have undergone modification several times to adapt to changes in research and our view of the ethical issues raised by it. In addition to these guidelines, the Council for International Organizations of Medical Sciences (CIOMS) produced ethical guidelines for both clinical and epidemiological research in international settings. The International Conference on Harmonisation publishes a series of guidance documents intended to facilitate new-product registrations in Europe, Japan, and the United States by reducing the need to duplicate the testing carried out during the research and development of new medicines. Similarly, other professional societies, such as the American Anthropological Association and the American Sociological Association have developed ethics codes of practice that reflect the issues arising in social science research. Investigators should be familiar with the codes impacting their research and understand their application.