Autologous conditioned serum was developed in the mid 1990s as an expeditious, practical, and relatively inexpensive means of generating the interleukin-1 receptor antagonist, a naturally occurring inhibitor of the cytokine interleukin-1. The latter is thought to be an important mediator of inflammation, pain, and tissue destruction in musculoskeletal conditions. ACS has been widely and successfully used in the local treatment of human and equine osteoarthritis and radicular compression; it has also shown promise in treating tendinopathies, muscle injuries, and tunnel widening after reconstruction of the anterior cruciate ligament. Experience suggests that autologous conditioned serum is safe and effective.