Eccentric muscle actions regularly occur as a braking or opposition force to a concentric (shortening) actions in many movements, in order to protect joint structures from damage. With an eccentric action the muscle elongates while under tension, due to an opposing force (such as a weight) being greater than the force generated by the muscle. Most of the classical muscle load studies in exercise physiology have focused on isometric (same length) and isotonic (shortening) contractions. Nonetheless, one of the first research observations with eccentric muscle actions was examined in 1882 by Fick, when he discovered that a contracting muscle under stretch could produce a greater force than a shortening muscle contraction (Lindstedt, LaStayo, and Reich, 2001). About fifty years later, A.V. Hill (who became a Nobel laureate) ascertained that the body had a lower energy demand when doing an eccentric muscle action as compared to a concentric muscle action (Lindstedt, LaStayo, and Reich). According to Lindstedt, LaStayo, and Reich, in 1953 Asmussen introduced eccentric exercise as "excentric”, with 'ex' meaning away from, and centric referring to center, thus giving the meaning of moving away from center. Lindstedt and colleagues further explain that when the weight exceeds the force developed by the muscle, as in an eccentric muscle action, it is referred to as 'negative work”, because the muscle is absorbing energy in this loaded motion. Research in the field of eccentric exercise is continuing to expand in many areas of sports and rehabilitation. This review will examine the physiological mechanisms of eccentric exercise, the effects of eccentric exercise on DOMS, the repeated bout effect, unilateral eccentric exercise and its effects on the unmovable limb, differences in the response of old and young persons to eccentric training, submaximal versus maximal eccentric exercise and the effects on muscle damage, eccentric exercise and 1-RM strength,eccentric exercise and rehabilitation, and the energy costs of eccentric exercise.