Analgesia is defined as loss of pain sensation without loss of consciousness; pain may be acute or chronic. Acute pain is well understood and can be controlled with currently available analgesics. Chronic pain, however, is not effectively controlled with current analgesics and side effect profiles often limit the use of these agents. Currently there are: (i) no objective methods for defining pain or analgesia in humans; (ii) no objective methods for correlating efficacy of analgesics in animal testing with human testing; and (iii) no objective method of evaluating the efficacy of analgesics in painful conditions, including neuropathic pain in which adaptive or maladaptive changes evolve with time. A technological revolution in functional brain imaging in humans and animals offers new approaches to objective evaluation of analgesics and of clinical pain states. These approaches hold great promise for revolutionizing drug development at preclinical and clinical stages.