The emerging field of molecular epigenetics studies relatively stable changes in genetic activity that are not due to changes in the DNA sequence. Initial research results indicate a functional role for epigenetic mechanisms in neuron development and neuronal cell function. However, concepts that integrate these findings in an overall theory of psychobiological development have yet to be developed. Gilbert Gottlieb's probabilistic epigenesis and his scheme of psychobiological development provide an ideal starting point for this effort. Based on a modified version of Gottlieb's scheme of psychobiological development, this study conceptualizes epigenetic mechanisms as molecular underpinnings mediating interactions between neural and genetic activity levels. Three different functional contexts of epigenetic mechanisms in neuronal cells are identified: genomic, developmental, and synaptic. Finally, consequences for a model of psychobiological development and implications for future research in molecular epigenetics are discussed.