Aquatic environments act as the ultimate recipients for a large number of chemicals capable of inducing DNA damage, originating from multiple sources. Therefore, genotoxicity tests are gaining importance as the production and release of chemicals with potential toxicity increase. This chapter presents an overview of the use of fish as a model organism in aquatic genotoxicity studies and provides a guide for measuring genotoxic damage in fish. The most commonly used genotoxicity endpoints, the Comet assay and the micronucleus test, in different fish tissues as well as erythrocyte nuclear abnormalities are described in detail, including methodology. Special emphases have been given to study designs for both laboratory and field conditions.