Physical activity is associated with reduced risk of depression, heart disease, stroke, type 2 diabetes and many forms of cancer. Physical inactivity has been estimated to cost the NHS £1.06 billion per year in direct costs, with a further £6.5 billion in lost work productivity. Thus, inactivity is a major drain on the physical, mental and economic health of the UK and understanding the key factors that influence physical activity is the critical first phase of developing strategies to change behaviour at the population level. This application focusses on assessing the physical activity patterns of young adults (age 28-30) in the ALSPAC cohort and the aspects of their current life and their development that help to explain why they adopt their current pattern of physical activity behaviour. This is potentially a very important time for the establishment of future disease risk but for which there is a lack of information on physical activity or how to transition to adulthood well