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. This includes information 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 a chapter providing a longitudinal case illustration of how the measure and the short forms of the GMFM can be applied and interpreted in clinical practice.

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).

Dianne J Russell is Research and Knowledge Exchange Consultant and Professor Emeritus with CanChild Centre for Childhood Disability Research. Dianne has been a health services researcher for over 30 years and a key individual in the development, evaluation and dissemination of clinical outcome measures such as the Gross Motor Function Measure (GMFM). Her consulting is focused on facilitating the use of research evidence in practice by engaging with research users throughout the research process and by making research results easily accessible in multiple formats to families, service providers, and policy decision-makers.

Marilyn Wright is Clinical Consultant, CanChild Centre for Childhood Disability Research, Assistant Clinical Professor, McMaster University, Physiotherapist, McMaster Children's Hospital. Marilyn is a physiotherapist who has worked in paediatric rehabilitation, research and education. This has included the development of Gross Motor Function Measure education and evaluation materials.

Peter Rosenbaum is Canada Research Chair in Childhood Disability Co-Founder, CanChild Centre for Childhood Disability Research, a Developmental Paediatrician, health services research, teacher, writer and editor. His career has been devoted to childhood disability, and all his roles have involved work with, or about, children with impairments and their families. He has held over 75 research grants and has contributed to almost 300 peer-reviewed papers, book chapters, editorials and invited commentaries in his field.

Lisa Avery is a statistical consultant specialising in longitudinal modelling and measurement development.