Cancer is the second leading cause of death in 0-14 year olds in the US and the sixth most prevalent chronic disease of childhood (behind asthma, congenital heart disease, epilepsy, cerebral palsy, and diabetes). Ionizing radiation and chemotherapies are currently the only external environmental substances clearly demonstrated to cause childhood leukemia. Other speculated agents include: electromagnetic fields (EMFs), radon, pesticides, solvents, diet, environmental tobacco smoke, alcohol, and infection. Additional research is underway to determine the potential importance of in utero and/or post natal exposures to toxicants such as pesticides, solvents, radiation, electromagnetic fields, indoor radon, food additives, and prescription medicines.