In recent decades the sports opportunities available to young athletes has risen significantly. The effect of such high levels of physical exertion on the developing skeleton is largely unknown. Retired football players have significantly greater rates of hip osteoarthritis than control groups, and a higher prevalence of morphological abnormalities known to predispose to future osteoarthritis, such as cam morphology, acetabular dysplasia and retroversion, and varus leg alignment. Cam morphology is likely acquired during adolescence, and dysplasia and leg alignment develop from birth throughout childhood and adolescence. High activity is a recognised risk factor for the development of these morphologies. However, their aetiology is insufficiently understood to recommend intervention. The aim of this thesis is to explore the impact of high levels of activity during adolescence on the development of the skeletally immature hip and knee. This is achieved via a longitudinal prospective observational cohort study in elite level footballers and age matched controls age 9-18 years. The first three studies in this thesis investigate the effect of activity on hip morphology and leg alignment. The results provide strong evidence that elite level football and high activity during adolescence significantly influences skeletal development in these areas. High activity individuals are more likely to develop cam morphology, dysplastic and retroverted acetabula, and varus leg alignment. The most significant effects appear to occur between 11 and 14 years of age. At present, it is not possible to recommend intervention such as activity modification due to our limited understanding of aetiology, and the cardiovascular benefits of exercise are likely to outweigh potentially detrimental effects on morphology. However, high activity individuals between the ages of 11 and 14 years likely represent a high-risk cohort warranting surveillance for early joint failure. The effect of exercise on cartilage in weight bearing joints in young healthy individuals is poorly understood. The final study in this thesis uses compositional MRI to determine the effect of high activity on cartilage composition during youth. The results suggest that elite level footballers and individuals with high activity levels may have differing cartilage composition and architecture to low activity individuals during adolescence.