One of the most enduring dichotomies in psychology is the "nature vs. nurture" debate. The human gene project allows us to identify every gene in the human (and mouse, rat, fruitfly, etc) and to understand the contributions of genes to neural and behavioural development. We are also able to identify the physical, social and chemical factors which influence neuro-behavioural development. Epigenetics is the study of how environmental variables regulate gene expression. The action of environmental factors on neurochemical systems and intracellular transduction pathways of neural cells modify the structure of histone proteins in the chromatin of the cell nucleus and these histone proteins regulate gene expression. Thus epigenetic processes provide the mechanism for environmental regulation of gene expression and the solution to the nature-nurture problem. This talk gives a brief outline of epigenetics and describes epigenetic approaches to the study of behavioural development, aging, learning, memory, social behaviour and neuropsychiatric disorders.