Delayed-onset muscular soreness (DOMS), the sensation of pain and stiffness in the muscles that occurs from 1 to 5 d following unaccustomed exercise, can adversely affect muscular performance, both from voluntary reduction of effort and from inherent loss of capacity of the muscles to produce force. This reduction in performance is temporary; permanent impairment does not occur. A number of clinical correlates are associated with DOMS, including elevations in plasma enzymes, myoglobinemia, and abnormal muscle histology and ultrastructure; exertional rhabdomyolysis appears to be the extreme form of DOMS. Presently, the best treatment for DOMS appears to be muscular activity, although the sensation again returns following the exercise. Training for the specific contractile activity that causes DOMS reduces the soreness response. The etiology and cellular mechanisms of DOMS are not known, but a number of hypotheses exist to explain the phenomenon. The following model may be proposed: 1) high tensions (particularly those associated with eccentric exercise) in the contractile/elastic system of the muscle result in structural damage; 2) cell membrane damage leads to disruption of Ca++ homeostasis in the injured fibers, resulting in necrosis that peaks about 2 d post-exercise; and 3) products of macrophage activity and intracellular contents accumulate in the interstitium, which in turn stimulate free nerve endings of group-IV sensory neurons in the muscles leading to the sensation of DOMS.