This project will investigate gene environment interactions in body mass index (BMI). We will test whether or not children at high risk of obesity due to their genes are at even higher risk of obesity if they are inactive. Importantly we will test whether or not the genetics and the activity add up to greater than the sum of their parts. In other words, if BMI-genes add half a stone of weight, and inactivity adds half a stone in weight, are children who are at high risk from their genes and are inactive, one stone or more than one stone heavier ? We will take carefuk steps to ensure that the BMI genes we study are not altering activity directly, but instead focus on those altering appetite. To address our question we plan to use a set of genetic variants known to be associated with BMI and objective measures of physical activity derived from the accelerometer devices worn by 5500 ALSPAC children and 2000 mothers.