The Gross Motor Function Classification System (GMFCS) for cerebral palsy has been widely used internationally for clinical, research, and administrative purposes. This paper recounts the ideas and work behind the creation of the GMFCS, reports on the lessons learned, and identifies some philosophical challenges inherent in trying to develop an ordered, valid, and consistent system to describe function in children and adolescents with developmental differences. It is hoped that these ideas will be useful to others who choose to expand the field with additional systems in other areas of childhood neurodisability.