In an influential book published more than twenty years ago, Archie Cochrane drew attention to our great collective ignorance about the effects of health care, and explained how evidence from randomized controlled trials (RCTs) could help us to use resources more rationally. He recognized that people who want to take more informed decisions about health care do not have ready access to reliable reviews of the available evidence. In 1979, he wrote: "It is surely a great criticism of our profession that we have not organised a critical summary, by specialty or subspecialty, adapted periodically, of all relevant randomised controlled trials." The Cochrane Collaboration has evolved in response to this challenge and will eventually cover all areas of health care. Contributors in many countries and specialties are preparing and maintaining systematic reviews of RCTs, and reviews of other evidence when appropriate. These reviews will be disseminated using electronic media through the Cochrane Database of Systematic Reviews.