An estimated 1 of 3 Americans uses some form of complementary and alternative medicine (CAM), such as acupuncture, homeopathy, or herbal medicine. In 1995, the National Institutes of Health Office of Alternative Medicine convened an expert panel to examine the role of clinical practice guidelines in CAM. The panel concluded that CAM practices currently are unsuitable for the development of evidence-based practice guidelines, in part because of the lack of relevant outcomes data from well-designed clinical trials. Moreover, the notions of standardization and appropriateness, inherent in guideline development, face challenging methodologic problems when applied to CAM, which considers many different treatment practices appropriate and encourages highly individualized care. Due to different belief systems and divergent theories about the nature of health and illness, CAM disciplines have fundamental differences in how they define target conditions, causes of disease, interventions, and outcome measures of effectiveness. These differences are even more striking when compared with those used by Western medicine. The panel made a series of recommendations on strategies to strengthen the evidence base for future guideline development in CAM and to meet better the current information needs of clinicians, patients, and guideline developers who seek information about CAM treatments.