Bacterial mutagenicity assays have been widely used in genotoxicology research for two decades. We discuss the development of such assays, especially the Ames test, with particular attention to strain engineering. Genes encoding enzymes of mutagen bioactivation, including N-acetyltransferase, nitroreductase, and cytochrome P450, have been introduced into tester strains. The processing of DNA damage by the bacterial strains has also been modified in several ways, so as to enhance mutagenesis. These efforts have greatly increased the sensitivity of mutation assays and have illuminated the molecular mechanisms of mutagenesis. We also discuss the relationship between bacterial assays and in vivo mutation assays which use transgenic rodents.