Epigenetics refers to the regulation of gene activity that does not lead to a change of DNA sequence. Gene expression is epigenetically controlled via heritable, but potentially reversible, changes in modification of DNA molecule and histone proteins. Those changes include DNA methylation and histone tails modification such as phosphorylation, acetylation, methylation, ubiquitination or ADP-ribosylation. Epigenetic alterations are more dynamic that alterations in DNA sequence, and under the influence of the environment. Studies of effect of epigenetic alterations involved in psychiatric diseases lead to better understanding of these diseases and development of new, improved therapies. Most of all, HDAC and DNA methyltransferase inhibitors have been highlighted. Studies had shown that aberrant expression of gene candidate, present in disease, can be restored back to normal level present in healthy people by administration of these drugs. Therefore, it can be concluded that environment affects pathophysiology of psychiatric diseases by changes of gene expression mediated by epigenetic mechanisms.