The unbridled enthusiasm for guidelines, and the unrealistic expectations about what they will accomplish, frequently betray inexperience and unfamiliarity with their limitations and potential hazards. Naive consu­ mers of guidelines accept official recommendations at face value, espe­ cially when they carry the imprimatur of prominent professional groups or government bodies. However, a growing awareness of their limitations and harms has done little to stem the rapid promulgation of guidelines around the world. It is therefore appropriate to begin a book on clinical practice guidelines by considering their potential benefits and harms.