Practice guidelines that specify how to treat medical conditions and perform procedures are appearing with greater frequency in the medical literature. Their use by managed care plans, hospitals and government programs is expected to affect the practice of medicine substantially in the coming years. This article reviews the key information that family physicians should have in order to evaluate and use practice guidelines effectively: how they are developed; how they differ from textbooks, review articles and other sources of expert consultation; whether they promote "cookbook medicine"; when to modify one's clinical practice in response to new guidelines, and how to cope with conflicting recommendations. Practice guidelines can improve the quality of care by summarizing current evidence and expert opinion, but they can also reduce the quality of care if the recommendations are poorly supported by scientific evidence and clinical reasoning. Economic and medicolegal concerns can also influence the potential benefits and harms of practice guidelines. Since hundreds of practice guidelines are anticipated to be developed in the coming years, family physicians should become informed consumers of guidelines, avoid accepting them on face value, and ask specific questions to judge their quality.