Damage to DNA appears to be the major cause of most cancer and genetic birth defects and may contribute to aging and heart disease as well. The agents that cause this damage must be identified. Many of these agents are natural chemicals present in the human diet as complex mixtures. The tens of thousands of man-made chemicals that have been introduced into the environment in the last few decades must also be tested for their ability to damage DNA. Existing animal tests and human epidemiology alone are inadequate for this task because of time, expense, and the difficulty of dealing with complex mixtures, Newly developed short-term tests, most of them assaying for mutagenicity, are discussed as key tools in identifying environmental mutagens and carcinogens.