This article reports a study of the activities and performance of institutional review boards to protect human research subjects. Researchers and institutional review board members were generally supportive of the review system, although substantial criticisms were also heard. Institutional review boards had some direct impact on half of the proposals reviewed by requiring either modification of or additional information about proposed research. The data, however, raise questions about the effectiveness of some review board actions, for example, with regard to informed consent. Some policy implications of the study are presented.