DNA methylation is an epigenetic phenomenon whereby a methyl (-CH3) group is added to cytosine. There are two basic types of methylation: hypomethylation in which the level of DNA methylation is greatly reduced, and hypermethylation in which there is an increased level. A methyl group is added and removed by DNA methyltransferases (DNMTs) and DNA demethylases respectively. The methyl group is transferred from a universal methyl donor S-adenosyl methionine. DNA hypermethylation in the promoter element can repress gene expression and therefore is crucial for a wide range of cellular activities such as genome stability and protection, imprinting, X-chromosome inactivation, paramutation, tissue specific gene regulation, carcinogenesis and aging. Global DNA hypomethylation has been known to play significant role in carcinogenesis. Studies in the field of DNA methylation have yielded promising potential disease biomarkers in terms of therapeutic functions. It has been concluded that the possible utilization of the degree of DNA methylation of specific genes as biomarkers for the prognosis and diagnosis of diseases is a matter which demands consideration for researchers all over the world. Canonical information on DNA methylation is highlighted in this review.