The chapter cites examples of the common methods used to determine the toxicity of chemicals to bacteria. It covers only the most common methods, particularly those that are easy to perform. Numerous literature citations have been included to help illustrate how a method is used and where its advantages and disadvantages lie. The information presented here is not a complete survey, but rather an aid to understanding of current methods used in the field of microbial toxicology. Toxicity of chemicals to bacteria is discussed relative to the organisms' growth and metabolism. Examples are given of methods involving heterotrophic activity, geochemical cycling, and decomposition processes. In addition, application of toxicity studies to unique bacterial processes and habitats discussed.