The Gross Motor Function Classification System (GMFCS) was developed in response to the need to have a standardized system to measure the ‘severity of movement disability’ in children with cerebral palsy (CP). The Surveillance of Cerebral Palsy in Europe classification ably describes the primary neurological impairment, subtypes, and topography of body involvement but currently includes non-standardized estimations of the degree of functional limitation. The reliability of other classification systems that measure functional ability such as ‘mild’, ‘moderate’ and ‘severe’, or ‘restricted walking’, ‘walking with aids’ or ‘non-walking’, or ‘household and community ambulator’ have not, to our knowledge, been evaluated. Palisano and colleagues used the underlying construct of self-initiated functional abilities in sitting and walking and the need for assistive devices, such as walkers or wheelchairs, to develop a new system, and set out to test its validity and reliability. The GMFCS now provides a method of describing the functional ability of children with CP in one of five levels. Children in Level I can perform all the activities of their age-matched peers, albeit with some difficulty with speed, balance, and coordination; children in Level V have difficulty controlling their head and trunk posture in most positions and in achieving any voluntary control of movement. Over the past few years the developers of the GMFCS have provided further evidence of the measurement properties of the system and it is being widely used. Here we systematically review publications that have cited the GMFCS, to describe the state of our knowledge about its properties and to describe its impact in the fields of research, clinical practice, and education to enhance our understanding of children with CP.