The WHO/International Programme of Chemical Safety Guidelines for Human Monitoring provide concise guidance on the planning, performance and interpretation of studies to monitor groups or individuals exposed to genotoxic agents. Most human carcinogens are genotoxic but not all genotoxic agents are carcinogenic in humans. Although the main interest in these studies is due to the association of genotoxicity with carcinogenicty, there is also an interest in monitoring human genotoxicity independently of cancer as an endpoint. The most often studied genotoxicity endpoints selected for inclusion in this document are described with the exception of germ cells [1]. It is clear from results of international collaborative studies that no single assay can detect all genotoxic substances. In 1996, WHO/IPCS published its Harmonised Scheme for Mutagenicity testing and this has now been updated [2] and focuses on the identification of mutagens and genotoxic carcinogens. It considers appropriate in vitro and in vivo assays and a strategy for germ cell testing. A combination of tests assesses effects of the three major end-points of genetic damage which are associated with human disease (gene mutation, clastogenicity and aneuploidy). Most assays in bacteria and mammalian cells are used for hazard identification. When choosing in vivo assays, either in somatic or germ cells, expert judgement is required for the appropriate test system and the unnecessary use of animals. Germ cell tests measure damage in germ cells per se and effects in the offspring or potential offspring of exposed animals.