Acupuncture (AP) is effective for the treatment of postoperative and chemotherapy-induced nausea/vomiting and for postoperative dental pain. Several recent randomized trials have provided strong evidence for beneficial AP effects on chronic low-back pain and pain from knee osteoarthritis. For many other chronic pain conditions, including headaches, neck pain, and fibromyalgia, the evidence supporting AP's efficacy is less convincing. AP's effects on experimental pain appear to be mediated by analgesic brain mechanisms through the release of neurohumoral factors, some of which can be inhibited by the opioid antagonist naloxone. In contrast to placebo analgesia, AP-related pain relief takes considerable time to develop and to resolve. Thus, some of the long-term effects of AP analgesia cannot be explained by placebo mechanisms. Furthermore, it appears that some forms of AP are more effective for providing analgesia than others. Particularly, electro-AP seems best to activate powerful opioid and non-opioid analgesic mechanisms.