The term nature versus nurture was in its modern sense coined by the English Victorian polymath Francis Galton in the discussion of the influence of heredity and environment on social skills. Nature versus nurture can be translated to genetics versus environment. Recent research suggests that part of the environments impact on humans is via epigenetic mechanisms. The term epigenetics refers to changes in gene expression, which does not involve any change in the DNA sequence. Epigenetic modifications can lead to genes being activated or deactivated, the modifications can be long or short term, and in some cases even life long and over several generations. The aim of this thesis is to give a brief introduction to epigenetic mechanisms, and to give a taste of how epigenetic modifications are associated with certain types of behavior. More specifically, this thesis describes epigentic modifications that have been observed to influence maternal behavior, stress behavior, cognitive abilities and substance abuse. In addition, it describes a couple of examples of transgenerational epigenetics; epigenetic alterations that are retained over generations – apparently hereditary alterations. Among the findings of this litterature study is that rat offspring of less caring rat mothers (measured by the frequency of LG-ABN) scores higher on stress behavior, and lower on maternal care skills and cognitive abilities through altered function of respectively the HPA axis, postpartum oxytocin receptors and NMDA receptors. Cocaine and alcohol use in rats has been shown to cause permanent epigenetic modifications in brain reward, motivation and pleasure centers. Sibling and twin studies have concluded that stress behavior, cognitive abilities and substance abuse are primarily affected by genetics and less by the environment. Research has, however, failed to identify single genes with major effect on such behavior, which indicates a polygenetic explanation. The Dutch famine birth cohort study and two Swedish cohort studies have shown that epigenetic changes may persist over several generations. The development of epigenetic research methods, including the ENCODE project, can potentially contribute to changing the future human medical diagnostics and therapy, as well as increase understanding of what determines our behavior.