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To review the mechanisms for the production of pain in knee osteoarthritis.Nociception is produced by stimulation of unmyelinated and small myelinated fibers in the joint and surrounding tissue. To produce pain, the stimuli must be either repeated or spatially clustered. When they reach the spinal cord, stimuli are subject to two inhibitory effectors: interneurons and descending central neurons. Inflammation lowers the threshold for nociception. In the joint, tissues containing nociceptors include primarily the joint capsule, ligaments, synovium, bone, and in the knee, the outer edge of the menisci. Nociceptive stimuli are likely to emanate from one or more of these locations in people with knee pain.This review does not cover psychological aspects of pain.Nociception in the knee is complex, and the nociceptive stimuli are related to but fundamentally different from those producing cartilage loss. Better appreciation for these processes will facilitate the development of new treatments.