Polyphenols are secondary plant metabolites that show positive effects on various human diseases, including tumors. Tumor appears as a result of genetic and epigenetic changes, and polyphenols can affect the inhibition of tumor growth by altering the reversible epigenetic modification. This paper describes the effect of polyphenols on the aberrant epigenetic modification in tumor cells. Polyphenols have been shown to inhibit DNA methyltransferase which results in inhibition of DNA methylation and activation of transcriptional activity. They also demonstrated the effect on histone modification (acetylation and methylation) where activation or inhibition of involved enzymes (histone acetyl- and methyl-tranferase, histone deacetylase and demethylase) can regulate the chromatin state and with that a transcriptional activity. The third model of regulation of gene expression has been shown to reduce or increase the expression level of different miRNAs, which then influence the expression of other factors involved in carcinogenesis. Through these three mechanisms it is possible to regulate transcriptional activity which is essential for the modification of improper expression of tumorsuppressor genes and oncogenes in tumor cells. By silencing of the oncogenes and the reactivation of epigenetically silenced tumor-suppressor genes polyphenols inhibit tumor growth. Although providing good results in in vitro conditions, polyphenols can not yet be used for therapy because of non-specific action on enzymes, tissue-specific epigenetic modification and lack of bioavailability in the living organism. If these problems are solved, polyphenols represent the future of chemoprevention and tumor therapy.