To increase young people's physical activity (PA) levels it is important to understand the correlates of PA in children and adolescents. We sought to identify factors associated with children's and adolescents’ PA by reviewing systematic quantitative reviews of non-intervention research. Systematic reviews examining associations between quantitatively measured variables and PA in young people (< 19 years) from 2000–2010 were identified using electronic and manual searches. Nine systematic reviews were identified. Demographic/biological correlates of PA were age and gender. Psychological correlates of PA were positive motivation, positive body image and the existence of barriers to PA. Behavioural correlates of PA were previous PA, sport participation, smoking, and sedentary behaviour. Social/cultural correlates of PA were parental influence and social support, and environmental correlates of PA were access to facilities, distance from home to school, time spent outside, and local crime. The evidence is suggestive of a number of different types of correlates of PA for children and adolescents. Beyond age and gender, though, most are likely to have only small or small-to-moderate effects in isolation and may work best in interaction with other influences. Psychologists must look to social, organisational and community-level correlates in addition to individual correlates.