Peer review can be performed successfully only if those involved have a clear idea as to its fundamental purpose. Most authors of articles on the subject assume that the purpose of peer review is quality control. This is an inadequate answer. The fundamental purpose of peer review in the biomedical sciences must be consistent with that of medicine itself, to cure sometimes, to relieve often, to comfort always. Peer review must therefore aim to facilitate the introduction into medicine of improved ways of curing, relieving, and comforting patients. The fulfillment of this aim requires both quality control and the encouragement of innovation. If an appropriate balance between the two is lost, then peer review will fail to fulfill its purpose.