This chapter discusses the most common disabilities, the nature of the physical constraints found in those disabilities, and the impact on movement skill and performance. The chapter examines historically the idea of physical constraints as a means of discussing motor skill development. It is only recently that a constraints perspective—especially a physical constraint perspective—has gained popularity among researchers in the area. Two major research approaches have been used in examining the relationship between intrinsic physical constraints and movement performance in non-handicapped children. First, correlational studies have evaluated the degree of relationships between body size, physique, strength, and motor performance in children. However, the second research approach—the direct manipulation of a morphological variable— allows for inferences regarding cause and effect.