The third edition of the Gross Motor Function Measure (GMFM-66 & GMFM-88) User’s Manual has retained the information contained in the original 2002 and 2013 publications which included the conceptual background to the development of the GMFM, and the administration and scoring guidelines for people to be able to administer this clinical and research assessment tool appropriately. Information is presented on the development and validation of two abbreviated methods of estimating GMFM-66 scores using the GMFM-66-Item sets (GMFM-66-IS) and the GMFM-66-B&C (Basal & Ceiling), and updates are included on the Challenge module and the Quality FM, translations, and the use of the GMFM in populations other than cerebral palsy and Down syndrome. The new edition includes information and an Appendix on the updated version of the Gross Motor Ability Estimator scoring program (GMAE-3), which is available through the GMFM App+ (see the CanChild website at (https://www.canchild.ca/en/shop/38-the-gross-motor-function-measure-app).