[Truncated abstract] Introduction: Childhood obesity is currently a serious worldwide public health issue associated with many medical and psychosocial complications. The increasing disease burden with the potential for the development of medical co-morbidities has implications for future health care provision. This thesis adds to the understanding of the medical complications of overweight and obesity in childhood. Design and Aims: Two different, but related, research studies are reported. The first study is a cross-sectional study, designed to quantify the medical complications of childhood obesity in primary school-aged children in Western Australia. This study aims to identify the medical complications of primary school children with overweight/obesity. The study also aims to compare the medical complications of obesity in a community sample who have never sought treatment with a clinical sample who are actively seeking treatment for overweight/obesity. Finally, this study also aims to examine the relationship between the medical complications of childhood obesity and a continuum of children's Body Mass Index z-scores, including those in the normal range. The second study is an exercise intervention study to investigate the effect of exercise on one specific medical complication of obesity, namely insulin resistance. This study aims to determine if a structured eight-week exercise program significantly changes insulin resistance in obese children, and to determine if this decrease in insulin resistance is associated with changes in body composition and inflammatory markers. ... Conclusion: The prevalence of the medical complications of overweight and obesity in primary school children indicates that all children should have body mass index regularly checked from a young age. Children who are overweight/obese should be screened for the presence of co-morbidities despite a young age. Parents and health professionals needs to be educated that childhood obesity is associated with medical co-morbidities and is not simply a social or cosmetic concern. The continuous nature of the BMI z-score/co-morbidities relationship suggests that public health and health education strategies should include adopting a populationbased approach to weight management. This continuous relationship means that even in the normal BMI spectrum, the risk of developing co-morbidities rises with increasing BMI. Such an approach would encourage maintenance of normal weight for all children, rather than targeting overweight/obese children only. Increased activity and decreased sedentary behaviours should be recommended for all children in line with the population-based public health approach suggested above. However, exercise has a specific role in weight management strategies for overweight/obese children, and in management strategies for adiposityrelated co-morbidities. Significant metabolic benefits of exercise occur in the absence of changes in body shape and weight. After an exercise program, simple blood investigations (such as lipid profiles, fasting insulin and OGTTs) are likely to miss important metabolic improvements and anthropometry (BMI calculation, waist circumference) may be more indicative of potential metabolic improvement and decreased co-morbidity risk.