Substances in the environment that are potentially harmful to children's health and development, from conception onward, include metals such as lead and mercury, pesticides, indoor air pollution, residues of synthetic chemicals, radiation from the sun, nuclear sources, and air pollution. Children's exposures to these substances, as well as prenatal exposure to "second-hand" smoke, alcohol, and drugs, are implicated in a multitude of health concerns, including: impaired growth and development; obesity; childhood cancers and other illnesses; chronic, sometimes lifelong, conditions such as asthma; and adult-onset disorders, including impaired fertility. Additional risk factors, including poverty, family violence, and inadequate access to health care, exist independently of environmental exposures. Children's environmental health is a relatively new and rapidly expanding field. Future research will determine more precisely how toxins (or combinations of toxins) contribute to or cause illness, which children are most susceptible, and how best to protect today's children and future generations.