A large number of studies have shown associations between birth weight and later adult disease, and these studies have ignited an interest in the developmental origins of disease and health. A paper in this volume of Epidemiology finds an overall U-shaped association between birth weight and all-cause mortality in a large Danish cohort. In this commentary, I discuss some of the issues that are important to epidemiologic studies concerned with the developmental origins of disease and health. These include considerations of causality and the public health/clinical relevance of the developmental origins of disease. I suggest that this area of research needs to move away from simply describing the association of birth weight with disease/health outcomes. Instead, we must aim to understand whether there are modifiable risk factors during the developmental period that are importantly causally related to later disease outcomes in ways that mean public health interventions should be aimed at the developmental period.