Exposure to environmental chemicals during development can result in fetal death in the most severe cases, or structural malformations and/or functional alterations in the embryo or fetus. The realization that developmental exposure to drugs and chemicals like diethylstilbestrol (DES) can cause permanent functional changes that are not overtly toxic like ionizing radiation or teratogenic like thalidomide, yet result in increased susceptibility to disease/dysfunction later in life, has led to a new field of toxicology called the developmental origins of disease. The DES episode is a salient reminder of the potential toxicity and carcinogenicity that may be caused by hormonally active chemicals if exposure occurs during critical windows of susceptibility. There is general agreement that the age of puberty is getting earlier for both boys and girls, and this has been attributed to exposure to environmental chemicals especially endocrine-disrupting chemicals (EDCs) during prenatal and/or early childhood life.