The Manual Ability Classification System (MACS) has been developed to classify how children with cerebral palsy (CP) use their hands when handling objects in daily activities. The classification is designed to reflect the child's typical manual performance, not the child's maximal capacity. It classifies the collaborative use of both hands together. Validation was based on the experience within an expert group, a review of the literature, and thorough analysis of children across a spectrum of function. Discussions continued until consensus was reached, first about the constructs, then about the content of the five levels. Parents and therapists were interviewed about the content and the description of levels. Reliability was tested between pairs of therapists for 168 children (70 females, 98 males; with hemiplegia [n=52], diplegia [n=70], tetraplegia [n=19], ataxia [n=6], dyskinesia [n=19], and unspecified CP [n=2]) between 4 and 18 years and between 25 parents and their children's therapists. The results demonstrated that MACS has good validity and reliability. The intraclass correlation coefficient between therapists was 0.97 (95% confidence interval 0.96–0.98), and between parents and therapist was 0.96 (0.89–0.98), indicating excellent agreement.