The aim of this paper is to review evidence of the effectiveness of interventions that present physical activity outcomes and potential mediators of behavioural change among 4-12-year-old children. A systematic search of electronic databases for original research articles published in peer-review journals between January 1985 and the end of June 2006 was carried out. A total of 19 studies that reported intervention effects on physical activity and mediators of behavioural change were identified. The most common mediators reported included physical activity knowledge or beliefs (11 studies); self-efficacy (8 studies); and enjoyment or preference for physical activity (6 studies). Less frequently reported mediators included attitudes, behavioural capability, intentions, outcome expectancies, social norms, social support and self-concept. Seven of the 11 interventions that reported intervention effects on knowledge/beliefs stated positive changes in this mediator. Four of the eight studies that reported intervention effects on self-efficacy had significant improvements; however, only two out of six interventions reported significant improvements in physical activity enjoyment or preference. None of the studies reviewed reported whether changes in these constructs mediated changes in children's physical activity behaviours. Although more than half of the studies reviewed reported a positive intervention effect on children's physical activity, no study carried out a mediating analysis to attempt to identify the mechanisms of change. Future research should more clearly identify the mediators of behavioural change that are being targeted and whether this explains intervention effects.