Since the rationing of scarce resources requires a targeting of those resources to obtain best value for money, it is important to have mechanisms for assuring effective health care. Clinical practice guidelines offer an opportunity for introducing evidence based health care into local practice and for influencing the commissioning of effective health care. The opportunities for guidelines to be influential are currently curtailed by developmental problems which have only recently been overcome. These problems and some of their solutions are discussed in a UK context, while using evidence from the considerable experience on guideline development in North America. In particular, the main attributes of guidelines are identified, the strengths and weaknesses of current guidelines, and their impact on patient care are considered. Overall, guidelines appear to have the potential for making a positive contribution to health care rationing through the better direction of resources and by limiting inappropriate variation in clinical practice.