There is clear evidence of socioeconomic inequalities in overweight and obesity in childhood. These differences progressively worsen with age ultimately amplifying disparities in morbidity and premature mortality from associated noncommunicable diseases across the life-course. The well-established benefits of physical activity during childhood in promoting health and reducing future disease risk has led to an international focus on its promotion across populations. Unlike in adults, it is unclear if the accumulation and distribution of physical activity in children differs by socioeconomic position. It furthermore is unknown whether all children benefit equally from current efforts to promote physical activity. In consideration of these evidence gaps, the aims of this thesis were: 1) to investigate the socioeconomic patterning of children’s physical activity behaviour, and 2) to explore if existing intervention efforts are generating differential effects. The findings from three secondary data analyses suggest that more socioeconomically affluent children accumulate a greater proportion of their daily physical activity from higher intensity activities, which are more strongly associated with lower levels of adiposity. This association was demonstrated both in the UK’s Millennium Cohort Study and South Africa’s Birth to Twenty Cohort. A subsequent analysis of pooled and harmonized data from 36 European cohorts with accelerometer-assessed physical activity confirmed these findings. It also revealed that irrespective of the national context, children with increased socioeconomic affluence engage in more vigorous physical activity and have lower adiposity despite overall lower levels of moderate-vigorous physical activity. Investigations of current efforts to promote physical activity were conducted through a two-stage systematic review and meta-analysis. An initial scoping review demonstrated an overall scarcity of published evidence on differential effects by sociodemo¬graphic characteristics. Subsequent meta-analyses of data re-analysed by authors revealed that current school-based physical activity interventions are not effective at increasing daily moderate-vigorous physical activity with no evidence of differential effectiveness by a child¬’s socioeconomic position or gender. This thesis demonstrates that socioeconomic differences exist in the intensity patterning of children’s physical activity behaviour with no evidence that current promotion efforts are propagating inequalities. The international focus on the aggregate of moderate-to-vigorous physical activity may be masking meaningful inequalities between socioeconomic groups of children. Further research is needed to determine the most appropriate daily dose of vigorous activity and to develop interventions that provide opportunities for less socioeconomically affluent children to engage in physical activity of a sufficient intensity to maximize health benefits. All physical activity promotion research should focus on the assessment and maximization of intervention fidelity.