Cancer is often viewed as a genetic process in which the developing cancer cell acquires successive mutational lesions that each provide the cell with a growth or survival advantage. The focus on genetic alterations in cancer research has perhaps led to an underestimation of the contribution by epigenetics. Epigenetic events are heritable alterations in gene function that are mediated by factors other than changes in primary DNA sequence; 5-methylcytosine DNA methylation is a good example. This article reviews current insights into the contribution of DNA methylation to mutational and epigenetic mechanisms of oncogenesis.