Epigenetic mechanisms provide a series of regulatory principles that define potentially stable and heritable changes in gene expression without altering the genetic information that is encoded within DNA. In understanding how epigenetic regulation defines cell type, it may also define patterns of gene expression that correlate with changes in behaviour and disease. The study of epigenetics reveals the complexity of regulatory mechanisms that control gene expression. This volume of Essays in Biochemistry provides an appraisal of the fundamental principles that facilitate the epigenetic regulation of chromatin function, with particular attention on the relevance to human disease and behaviour.