Traditionally, acupuncture is embedded in naturalistic theories that are compatible with Confucianism and Taoism. Such ideas as yin-yang, qi, dampness, and wind represent East Asian conceptual frameworks that emphasize the reliability of ordinary, human sensory awareness. Many physicians who practice acupuncture reject such prescientific notions. Numerous randomized, controlled trials and more than 25 systematic reviews and meta-analyses have evaluated the clinical efficacy of acupuncture. Evidence from these trials indicates that acupuncture is effective for emesis developing after surgery or chemotherapy in adults and for nausea associated with pregnancy. Good evidence exists that acupuncture is also effective for relieving dental pain. For such conditions as chronic pain, back pain, and headache, the data are equivocal or contradictory. Clinical research on acupuncture poses unique methodologic challenges. Properly performed acupuncture seems to be a safe procedure. Basic-science research provides evidence that begins to offer plausible mechanisms for the presumed physiologic effects of acupuncture. Multiple research approaches have shown that acupuncture activates endogenous opioid mechanisms. Recent data, obtained by using functional magnetic resonance imaging, suggest that acupuncture has regionally specific, quantifiable effects on relevant brain structures. Acupuncture may stimulate gene expression of neuropeptides. The training and provision of acupuncture care in the United States are rapidly expanding.