Obesity is a growing threat worldwide, and the prevalence has risen dramatically over the last decade. A number of epidemiological studies have shown that there is a direct relationship between birth weight and BMI in childhood and in adult life. A number of factors influence the development of childhood and adult obesity and birth weight as a proxy for the intrauterine environment may be one of the many. For example, a number of investigators have reported a significant increase in the rates of obesity in children, adolescents, and adults whose mothers had diabetes during pregnancy. A large number of studies have also linked low birth weight to the later development of central adiposity. Thus, both excess and reduced nutrient availability during fetal development can lead to the later development of obesity. This review summarizes both human and animal studies relating fetal exposures to later obesity.