The aim of this study was to validate the expanded and revised Gross Motor Function Classification System (GMFCS-E&R) for children and youth with cerebral palsy using group consensus methods. Eighteen physical therapists participated in a nominal group technique to evaluate the draft version of a 12- to 18-year age band. Subsequently, 30 health professionals from seven countries participated in a Delphi survey to evaluate the revised 12- to 18-year and 6- to 12-year age bands. Consensus was defined as agreement with a question by at least 80% of participants. After round 3 of the Delphi survey, consensus was achieved for the clarity and accuracy of the descriptions for each level and the distinctions between levels for both the 12- to 18-year and 6- to 12-year age bands. Participants also agreed that the distinction between capability and performance and the concept that environmental and personal factors influence methods of mobility were useful for classification of gross motor function. The results provide evidence of content validity of the GMFCS-E&R. The GMFCS-E&R has utility for communication, clinical decision making, databases, registries, and clinical research.