Childhood obesity has reached epidemic proportions worldwide and is associated with increased cardiovascular mortality and morbidity in adult life. The increase in fat mass in children and adolescents has occurred concomitantly with a decline in reported time for exercise. Evidence suggests that non-physically active children are more likely to become non-physically active adults and that encouraging the development of physical activity habits in children helps establish patterns that continue into adulthood. Dietary treatment of obesity is relatively ineffective in adults and it has been suggested that prevention of obesity in childhood and adolescence should emphasise increased physical activity rather than diet because of fears relating to the adverse effects of inappropriate eating patterns. Despite this, there are very few randomised controlled studies investigating the efficacy of exercise training in obese children or adolescents and many of the extant studies have been poorly controlled and have not specifically stratified the independent effect of exercise versus dietary modification. This review focuses on the well designed controlled trials that have evaluated the effect of exercise training in obese children and adolescents on body composition, haemodynamic and metabolic variables, cardiovascular fitness, muscular strength and vascular function. These studies indicate that, although exercise training does not consistently decrease bodyweight or body mass index, it is associated with beneficial changes in fat and lean body mass, emphasising the importance of comprehensive assessment of body composition in future exercise-training studies. Exercise training improves cardiovascular fitness and muscular strength; however, it seems to have little effect on blood lipid profile or blood pressure in obese young people. Importantly, recent studies have demonstrated that exercise training improves vascular endothelial function, an important surrogate measure that may predict future atherosclerotic risk in obese children and adolescents. Given that improvement in vascular function in these training studies occurred in the absence of changes in lipid fractions, haemodynamic variables or glucose metabolism, exercise appears to have a direct beneficial effect on the vasculature, in addition to its putative benefits through risk-factor modification.