Since the early 80's, cancer research has been dominated by scientific breakthroughs demonstrating the genetic origin of cancer. Thousands of genetic alterations have been identified, affecting more than one hundred cell regulating genes. In the past ten years, our understanding of carcinogenesis has evolved: cancer is both a genetic and an epigenetic disease. Epigenetic modifications play a fundamental biological role in the initiation and progression of cancer by altering the expression of cell cycle regulation genes. Unlike genetic mutations, epigenetic modifications are potentially reversible. Thus, epigenetic inhibitors are currently evaluated as anticancer drugs. Moreover, DNA methylation study holds promise as biological marker for classification, diagnostic and prognostic purposes in clinical practice.