Aim: This paper is a systematic review of physical activity measurement instruments for field-based studies involving children with cerebral palsy (CP). Method: Database searches using PubMed Central, MEDLINE, CINAHL Plus, PsycINFO, EMBASE, Cochrane Library, and PEDro located 12 research papers, identifying seven instruments that met the inclusion criteria of (1) having been developed for children aged 0 to 18 years, (2) having been used to evaluate a physical activity dimension, and (3) having been used in a field-based study involving children with CP. The instruments reviewed were the Activities Scale for Kids-Performance version (ASKp), the Canada Fitness Survey, the Children's Assessment of Participation and Enjoyment/Preferences for Activities of Children (CAPE/PAC), the Compendium of Physical Activities, the Physical Activity Questionnaire-Adolescents (PAQ-A), StepWatch, and the Uptimer. Second-round searches yielded 11 more papers, providing reliability and validity evidence for the instruments. Results: The instruments measure physical activity frequency, mode, domain, and duration. Although most instruments demonstrated adequate reliability and validity, only the ASKp and CAPE/PAC have established reliability and validity for children with physical disabilities; the Uptimer has established concurrent validity. No instrument measuring intensity in free-living has been validated or found reliable for children with CP. Interpretation: The findings suggest that further studies are needed to examine the methodological properties of physical activity measurement in children with CP. Combining subjective and objective instruments is recommended to achieve better understanding of physical activity participation.