Biomonitoring is a public health tool that has been used by scientists and researchers for decades to test blood, bone, urine, hair, human milk, adipose tissue, and other body substances for the presence of toxic chemicals, in order to assess what is called the "chemical body burden." Biomonitoring helps to: (1) identify which chemicals are accumulating in the population's bodies; and (2) determine if any of them, or combinations of them, is linked to preventable diseases. Advances in monitoring techniques in the past decade allow scientists to detect much lower levels of chemicals within human bodies than was previously possible. Scientists can now link low levels of exposure to health outcomes. By providing evidence of exposure, biomonitoring can help us learn more about disease causation. Nutrition, stress, individual genetic susceptibilities, in combination with chemical exposures, may all play roles in the incidence and course of many of the diseases U.S. families are experiencing today. The author reviews the implications of biomonitoring data for public policy. She endorses the principle of "No data, no market," according to which a chemical would be assumed to be highly hazardous unless comprehensive safety data are available for the chemical. Legislation would require manufacturers to provide this data by a certain deadline in order for the chemical to remain on the market.