Systematic reviews of the literature provide a summary of the current status of medical scientific research. They are important for the solution of medical questions by doctors active in clinical practice, may serve to support practice guidelines, and are used in health care to take policy decisions and to determine the research agenda. The Cochrane Library is the most important source of information on the efficacy of interventions in health care and comprises eight databases. The most relevant for doctors active in clinical practice are the Cochrane Database of Systematic Reviews, the Database of Abstracts of Reviews of Effects, and the Cochrane Central Register of Controlled Trials. At present, the first two databases contain over 5000 systematic reviews of the literature. The third database listed above is the largest database in the world with references to (randomised) controlled studies.