In this book, the genotoxic characteristics of synthetic reactive, synthetic disperse, synthetic direct and synthetic acid dyes are analysed by using UMU- TEST (Salmonella thyphimurium TA1535/pSK1002) system that is a short-term mutagenicity test. The status of the synthetic dyes after being metabolized in a living body is also researched with the assistance of used S-9 rat liver enzymes. Thus, it is stated a) whether dyes have genotoxicity characteristics directly, b) if so, whether they lose their genotoxicity characteristics with the treatment of enzymes in living bodies, and c) whether they gain genotoxicity characteristics with the treatment of enzymes in living bodies if they do not have genotoxicity.