Abstract Genetic toxicology is the science of how chemicals and radiations are toxic to/cause damage to the DNA of prokaryotic and eukaryotic cells. DNA repair, mutagenesis, and mutations that occur in bacteria and mammalian cells are discussed. Biological consequences of different mutations are detailed. Types of mutagens are listed. Metabolic activation of premutagens is introduced. Mechanisms of mutagenesis by radiations and chemicals are described. Assays to detect mutagens in bacteria/mammalian cells, micronuclei/chromosomal aberrations in mammalian cells, and chemically/radiation-induced morphological transformation in mammalian cells are described. Tier systems to detect genotoxic chemical carcinogens by measuring their genotoxicity are introduced.