Childhood obesity is a major public health problem because the prevalence is increasing and because childhood obesity is associated with short- and long-term adverse consequences. By contributing to our understanding of the causes of childhood obesity epidemiological studies can help to inform preventive strategies. Prospective studies with all-of-life measures of exposures, objective measures of physical activity and more accurate measures of diet and body composition will be better able to identify modifiable environmental exposures that act cumulatively or at critical time periods across the lifecourse. The Avon Longitudinal Study of Parents and Children (ALSPAC) recruited around 14 000 pregnant women with estimated dates of delivery between 1991 and 1992. The children have been followed-up in detail ever since and now constitute probably the most intensively studied cohort of children ever recruited. Recent analyses have identified important modifiable risk factors and further analyses based on more accurate measures of diet, activity and body composition should provide further insights.