Developmental therapies in children with cerebral palsy have been a traditional mainstay of treatment. Their use is part of mandated intervention programs in every state. Until recently, however, research into the effects of these interventions has been limited both in scope and quality. This paper reviews the status of research into the effects of developmental therapies, using physical therapy in cerebral palsy as the model. Recent studies have not demonstrated consistent treatment effects in cerebral palsy, but the quantity and scope of the research is insufficient to make global statements about treatment effect. Directions for future research should include multi-center clinical trials incorporating no-treatment control groups as well as single-subject studies to generate hypotheses for larger studies. A wide variety of parameters of the subject-treatment interaction may affect treatment outcome and should be investigated. These include treatment timing, intensity, parent involvement, treatment combinations, and the profile of the child's associated disabilities. Study cohorts at extreme risk for cerebral palsy can be identified through neuroimaging and may allow evaluation of very early treatment effects. Studies evaluating a wide range of measures, including functional and economic outcomes, are necessary if decisions about program support are to based on treatment effectiveness as well as cost.