Bacterial mutagenicity tests, specifically the Salmonella and E. coli reverse mutation (Ames) test, are widely used and are usually required before a chemical, drug, pesticide, or food additive can be registered for use. The tests are also widely used for environmental monitoring to detect mutagens in air or water. Their use is based on the showing that a positive result in the test was highly predictive for carcinogenesis. This chapter describes the Salmonella and E. coli tests, presents protocols for their use, and addresses data interpretation and reporting.