Joline E. Brandenburg, M.D., Physical Medicine and Rehabilitation, discusses the rationale for studying passive muscle stiffness, the effects of spasticity on muscle in children with cerebral palsy (CP), normal passive gastrocnemius stiffness in children who are developing typically, the effect of botulinum toxin on passive muscle properties in children with CP, and a novel concept regarding etiology of spasticity and abnormal muscle properties in CP.