Epigenetics refers to the study of heritable changes in gene expression that occur without a change in DNA sequence. In the last decade, it has been shown that epigenetic mechanisms provide an "extra" layer of transcriptional control that regulates genes expression. Three distinct mechanisms appear intricately related in initiating and sustaining epigenetic modifications: RNA-associated silencing, DNA methylation and histone modification. These mechanisms are critical components in the normal development and cell growth. DNA methylation is involved in transcriptional silencing of genes, regulation of expression of imprinted genes, a number of tumour suppressor genes in cancer and silencing of genes located on the inactive X chromosome. In this review, we are focused on the basic principles of DNA methylation as the main epigenetic mechanism for normal embryonic development and epigenetic alterations that contribute to carcinogenesis.