The comet assay or single-cell gel electrophoresis (SCGE) assay is now widely accepted as a standard method for assessing DNA damage in individual cells. It finds use in a broad variety of applications including human biomonitoring, genotoxicology, ecological monitoring and as a tool for investigation of DNA damage and repair in different cell types in response to a range of DNA-damaging agents. The comet assay should be eminently suitable for use in clinical practice since it is a relatively simple and inexpensive technique which requires only a few cells, and results can be obtained within a matter of hours. This method can be used in the study of cancer as well as in lifestyle and dietary studies. In cancer it is useful for measuring DNA damage before, throughout and after therapy (either radiotherapy or chemotherapy). Another use of this method is in lifestyle study, such as investigation of the effect on DNA of common human activities (e.g. smoking, or working with a potentially genotoxic agent). The final use of comet assay in this paper is dietary study. In this type of study we observe the effects of consumption of specific foods or supplements which may be protective for DNA against damage.