DNA methylation is one of several epigenetic changes observed in cells. Aberrant methylation of tumor suppressor genes, proto-oncogenes, and vital cell cycle genes has led many scientists to investigate the underlying cellular mechanisms of DNA methylation under normal and pathological conditions. Although DNA methylation is necessary for normal mammalian embryogenesis, both hypo- and hypermethylation of DNA are frequently observed in carcinogenesis and other pathological disorders. DNA hypermethylation silences the transcription of many tumor suppressor genes, resulting in immortalization of tumor cells. The reverse process, demethylation and restoration of normal functional expression of genes, is augmented by DNA methylation inhibitors. Recent studies suggest that DNA hypomethylation may also control gene expression and chromosomal stability. However, the roles of and relationship between hypomethylation and hypermethylation are not well understood. This review provides a brief overview of the mechanism of DNA methylation, its relationship to extrinsic stimulation including dietary intake and aging, and of abnormally methylated DNA in breast and colorectal cancers, which could be used as prognostic and diagnostic markers.