This project involves a series of inter-related research studies investigating the behavioural and sociodemographic predictors of weight gain and obesity among Australian adults. Specifically, the studies will investigate and quantify the relationships between weight gain and physical activity, sedentariness, and diet. The relationship between socioeconomic status and obesity will also be examined. Finally, the feasibility of an intervention program designed to prevent weight gain will be evaluated. The studies are designed to provide an evidence-base from which to develop intervention strategies to prevent weight gain and obesity in the population.