For medical decisions with more than one reasonable option, patient participation in decision making is often necessary to optimally match management decisions with patient preferences. Health decision aids are designed to facilitate shared decision making by helping patients and their physicians choose among reasonable clinical options. Although these aids vary in content, common denominators are the presentation of more than one reasonable strategy for a clinical management question and a description of the possible outcomes of the various options. Although the number of published randomized trials assessing the impact of health decision aids on the quality of medical decisions is limited (but growing), various types of decision aids do generally appear to inform patients about their treatment options better than "usual care" can. Little evidence is available to determine whether one type of decision aid is optimal, but more complicated programs seem to have larger effects. The cost-effectiveness of decision aids has not been studied, although it is enticing to think that the pattern of more conservative decisions by users of some decision aids could reduce medical care costs in a manner that is dictated by patient preferences.