The single cell gel (SCG) electrophoresis or comet assay is a technique detecting of multiple classes of DNA damage such as single and double strand breaks and oxidative base damage and is used for a wide range of applications. The comet assay is a relatively sensitive test which enables to measuring DNA damage spontaneously and caused by various agents. The principle advantages of the technique are apply to any cell type from prokaryotic and eukaryotic organisms, cultured cells or cells isolated from the organisms and it. The other advantages are its reliability, low costs, ease of application, and the short time needed to complete a study. Therefore, it has gained in popularity as a standard technique for evaluation of DNA damage/repair, biomonitoring and genotoxicity studies. The basic steps of the comet assay are preparation of microscope slides layered with cells in agarose, lysis, alkali treatment, electrophoresis under alkaline conditions, neutralization, DNA staining, comet visualization and comet scoring. Image analysis tools are available to analyze various parameters of the comets. The aim of this review is to give information about the principles and applications of the comet assay, a sensitive and rapid technique for the measurement of DNA damage