Epigenetic modifications include DNA methylation and covalent modification of histones. These alterations are reversible but very stable and exert a significant impact on the regulation of gene expression. Changes in methylation of promoter or first exon may mimic the effect of mutations of various tumor suppressor genes (TSGs) or protooncogenes. Carcinogenesis can also result from aberrations in genomic DNA methylation that include hypermethylation and hypomethylation of promoter or first exon of cancer-related genes. Hypermethylation of promoter of various TSGs causes their transcriptional silencing. However, hypomethylation of regulatory DNA sequences activates transcription of protooncogenes, retrotransposons, as well as genes encoding proteins involved in genomic instability and malignant cell metastasis. The methylation of genomic DNA in malignant cells is catalyzed by DNA methyltransferases DNMT1 and DNMT3B, revealing significantly elevated expression in different types of cancers. The reversibility of hypermethylation can be used as target of therapeutic treatment in cancer. DNMT 1 and DNMT3B inhibitors including 5-Aza-2'-deoxycytidine and antisense oligonucleotides have been applied in clinical trials of such treatment. Identification of aberrations of DNA methylation in cancer cells is a new field of investigation in carcinogenesis. We believe that epigenetic cancer diagnostic and therapy will be achieved in the next decades.