The “epi” prefix means "above in Greek." The literal translation of this prefix is "above the genome". Epigenetics encompasses processes that regulate DNA without changing its sequence. Specifically, there are over 200 different types of differentiated cells in the human body, all of which share the same genome. What links the cell genotype and the phenotype are the epigenetic mechanisms that decide which genes are active and to which extent they are expressed or silenced. The aim of this thesis is to explain the basic mechanisms of epigenetics. Basic epigenetic mechanisms for gene expression regulation, such as DNA methylation, histone modifications, and RNA interference, are outlined in this thesis, but it is important to note that they are not the only ones. This is why epigenetics is one of the fastest growing fields in modern biology. Its broad role in biological processes makes it extremely important in the pathophysiology of the disease. Accordingly, epigenetic pathophysiological mechanisms are present in diseases of many areas of oncology, neurology, psychiatry, immunology, embryology, etc. It is particularly interesting for scientists to investigate epigenetic changes in clinically challenging diseases, such as those of multifactorial origin and diseases of which treatment is limited in any way. In fact, epigenetically important proteins and molecular pathways have been studied aiming to find new targets and methods of treatment. So far FDA (Food and Drug Administration) had approved epigenetic drugs that are HDAC and DNMT enzyme inhibitors. These drugs are also called "first generation" drugs, but an increase in newly approved drugs is expected in the near future, especially those with the RNA interference activity.