Clinical guidelines are statements designed to help physicians make decisions about appropriate health care for specific circumstances. The constant rise in the number of published guidelines has been accelerated by the need of healthcare organizations to integrate evidence from clinical research with rational health policy, with the prospect of improving the quality and reducing the costs of health care at a local level. The best guidelines are developed from a systematic examination and appraisal of good evidence from well conducted trials, supported by appropriate clinical expertise, and leading to unambiguous recommendations. Great care needs to be taken both to maximize the validity of guidelines and to ensure their use within clinical practice. Moreover, the evidence on which clinical guidelines are based can change with time and therefore they should be reviewed regularly. The critical approaches to making high-quality guidelines, the value of implementation strategies, and how healthcare organizations and individual physicians can use medical guidelines to enhance clinical effectiveness will be discussed.