Much research has indicated that obesity is associated with poorer health outcomes. Whilst these studies have tended to investigate average body weight (relative to height), more recently there has been interest in variation in body weight. This work has indicated that people whose body weight goes up and down a lot are more likely to experience future weight gain. This project will investigate variability in weight, using body mass index (BMI, a measure of weight which accounts for height), across the key developmental phase of childhood through to early adulthood. It aims to form a better understanding of the factors associated with variability in BMI across this time.