Research often characterizes children and youth with physical disabilities as less physically active than their typically developing peers. To inform the development and evaluation of future interventions, it is important to identify the most accurate methods for assessing physical activity behavior in this population. The objectives of this review were 1) to identify the self-report and objective instruments used to examine habitual physical activity behavior within this population and 2) to determine the reliability and validity of these instruments. Following a standardized protocol, a systematic review was conducted using six electronic databases and a range of search terms. Fifty studies (N = 2,613; M_age = 11.3 ± 2.6 years; 53% male) were included. Seven disability groups were examined, with the majority of studies focused on cerebral palsy (64%) and juvenile arthritis (20%). Poor to good reliablity and weak validity were found among the self-report instruments such as questionnaires and activity diaries. Good to excellent reliability and validity were established for the objective instruments such as activity monitors (e.g., accelerometers, pedometers). Further research is warranted among physical disability groups other than cerebral palsy, and in establishing reliability and validity of self-report physical activity instruments specific to these target groups.