Aim: Examine the longitudinal association between the pattern of active travel to school throughout childhood and adolescence, physical activity and obesity. Hypothesis: Due to the substantial reduction in physical activity associated with changing from active to passive travel and the negative association of physical activity to adiposity (as shown in previous research), we believe this change should be associated with an increase in adiposity. Methods: To achieve this we will compare questionnaire data regarding mode of travel to and from school with objective measures of both physical activity and adiposity from 8 to 17 years of age. The exposure variables will be (a) travel mode as assessed by questionnaire at 8 years 7 months carer response, 9 years 7 months carer response, 11 years 8 months carer response, 13 years 10 months child response, and 16 years 6 months child response; (b) objectively measured physical activity by accelerometer. The outcome variable is obesity (fat mass) by dual energy x-ray absorptiometry (DXA). We will include previously identified determinants of active travel in the analysis: distance, neighbourhood safety and journey safety. We will consider the following as potential confounding variables: age, sex, social class (SES), mother's BMI, mother's highest education level, mother's smoking habits during pregnancy and pubertal status. These confounders are those available for the whole cohort and have been shown to be independently associated with obesity in previous research