The strategy for testing for genotoxicity covers three main areas, namely gene mutation, chromosome aberration or breakage (clastogenicity), and chromosome loss or gain (aneuploidy). The current generalized strategy consists of assays capable of detecting all of these endpoints using in vitro assays such as the Ames test for detecting gene mutations in bacteria, the human peripheral lymphocyte chromosome aberration (CA) test for detecting clastogenicity, and the in vitro micronucleus test for clastogenicity and aneuploidy. The primary in vivo assay, and generally the only in vivo assay required, is the in vivo rodent bone marrow micronucleus assay. However, there are instances when these assays alone are inadequate and further testing is required, especially in vivo. Historically, the preferred second assay has been the rodent liver unscheduled DNA synthesis assay but recently this has been superseded by the rodent single cell gel electrophoresis or Comet assay. This assay has numerous advantages especially in vivo, where virtually any tissue can be examined. The status of the in vitro comet assay in regulatory testing is much less clear although a preliminary review of data from the assay has shown it to be more specific than other in vitro genotoxicity tests and less prone to false positives.Detailed here are general protocols for both the in vitro and in vivo comet assays which will form the basis of the pending OECD guideline for the assay.