In a large number of studies, it has been demonstrated that organism is most sensitive to various environmental influences during early development. These influences lead to permanent changes in the structure and function of certain organs and systems. It is assumed that the basic molecular mechanism of such «ontogenetic programming» are epigenetic modifications (changes in genetic expression that are not accompanied by changes in DNA structure). The environmental stress can induce specific and predictable epigenetic changes that eventually result in an adaptive response to the stimulus. Early-life epigenetic adaptations usually are predictive and allow the organism to better suit the future environmental conditions. Where there is a match between the predicted and actual adult life conditions, these predictive adaptive responses are appropriate and assist survival. Where the prediction is incorrect, however, the organism is left with a postnatal physiology that is mismatched and inappropriate, putting it at increased risk of diseases. This review discusses the mechanisms of epigenetic regulation of gene expression, as well as their possible role in the ontogenetic programming and predictive adaptive response.