Obesity causes several chronic diseases, including cancer, diabetes and heart disease. When combined these conditions account for two thirds of all deaths in this country. Many children do not consume sufficient fruit and vegetables, and have levels of physical activity below recommended guidelines. Adolescence is a key period when young people start to make independent decisions about their lifestyle, typically with increased consumption of unhealthy foods and a drop in physical activity. To date little research has focussed on what it is specifically that encourages obesity. 1,000 children will be recruited during their first year of secondary school (i.e. 11-12 years old). The researchers will measure physical activity using activity monitors (which measure movement) and will track journeys to/from school and at lunchtime using Geographical Information Systems (GIS) and Global Positioning Satellite (GPS) receivers. This information will be cross-referenced with maps and will be used to trace the amount of physical activity undertaken and the number and type of food outlets visited over 7 days. This information will provide an objective measurement of how the children typically interact with their environment, while computerised questionnaires will measure the children’s personal, social and environmental conditions and physical activity. The same 1,000 children will be contacted two years later (aged 13-14) and will undergo the same testing. Time will be spent specifically looking at the relationship between the behaviour of the children and their associated obesity levels both at the start of the study and two years later. This research will provide important information about adolescent lifestyles in this country and will give a clear indication of the behaviours that typically lead to obesity. From these results the researchers will be able to design health interventions that promote better diets and higher activity levels in children.