The pathogenesis and clinical course of hip dysplasia in cerebral palsy and Down syndrome is different than idiopathic developmental dysplasia of the hip. Unlike idiopathic developmental hip dysplasia, hip development in cerebral palsy and Down syndrome is typically normal in utero and instability develops after birth secondary to musculoskeletal disorders associated with the disease condition. For this reason, treatment protocols in Down syndrome and cerebral palsy hip dysplasia differ greatly from protocols designed to treat idiopathic hip dysplasia. The purpose of this review is to describe the pathologic hip morphology that results from cerebral palsy and Down syndrome.