The overall goal of this research is to understand the mechanisms underlying the development of postural control in sitting using new methodology, in order to provide a scientific basis for evaluation and treatment of posture and movement disorders in infants with cerebral palsy. The development of early posture control remains poorly understood despite considerable therapeutic effort. Infants with cerebral palsy show their first delays in the acquisition of sitting, with subsequent problems developing adequate posture and movement control. Identifying the delay, determining the nature of the problem, and evaluating the effectiveness of treatment quickly, are vital in the early part of an infant's life, since this is the time of greatest plasticity. Tools from nonlinear dynamics, which are increasingly being used to examine other biological rhythms, are used in this study to analyze postural sway from center of pressure data during the development of sitting postural control.