Five medical conditions are responsible for approximately $250 billion in annual health care costs in the United States: obesity, asthma, diabetes, schizophrenia, and autism. For some individuals, these conditions may begin with in utero exposures. However, firm evidence about the links between these conditions and such exposures has yet to be established. The National Children's Study (NCS) is designed to examine how maternal health and the fetal environment are associated with these and other conditions, including birth defects. The NCS will assess how hundreds of social, physical, and environmental exposures affect the health of 100,000 children. The results will provide a data resource from which to develop effective preventive strategies, establish health and safety guidelines, find cures and interventions, influence legislation, and shape public health programs for families and children. The purpose of this article is to describe some of what is known about teratogenesis, how child and adult health can be affected by in utero exposures, and Minnesota's role in the NCS.