Hundreds of thousands of chemicals in current or past use are present in the environment, leaving human populations and ecosystems potentially at risk of exposure to them. The large number and various forms of chemicals preclude regulators from evaluating every chemical with the most rigorous testing strategies. Instead, standard toxicity tests have been limited to only a small number of chemicals, with the hope that the “worst” chemicals will receive specific attention. The chemicals that are tested may represent large classes of compounds, such as certain types of pesticides.