The comet assay has been widely used to measure a range of cellular responses to DNA damage and has found applications in genotoxicity studies, bio-monitoring, ecological testing and in the study of human disease. This review discusses how the comet assay has been applied to the study of DNA damage and repair associated with cancer. The potential of the assay as a tool for predicting an individual's tumour sensitivity to radiation and to various chemotherapeutic drugs is examined, as well as outlining the usefulness of the assay in assessing oxidative stress within tumours. In addition, we review the use of the comet assay in investigations of the DNA-damaging effect of anti-neoplastic drugs and radiation used during cancer therapy. The advantages and limitations of the comet assay in carrying out all these studies are outlined, and the suitability of the comet assay for use in the clinical management of cancer is discussed.