Epigenetics can be defined as all the meiotically and mitotically inherited changes in gene expression that are not encoded in the DNA sequence itself. Epigenetic modifications of chromatin and DNA have been recognized as important permissive and suppressive factors in controlling the expressed genome via gene transcription. Two major epigenetic mechanisms are the posttranslational modification of histone proteins in chromatin and the methylation of DNA itself, which are regulated by distinct, but coupled, pathways. It is clear that the epigenetic state is a central regulator of cellular development and activation. Emerging evidence suggests a key role for epigenetics in human pathologies, including in inflammatory and neoplastic disorders. The epigenome is influenced by environmental factors throughout life. Nutritional factors can have profound effects on the expression of specific genes by epigenetic modification, and these may be passed on to subsequent generations with potentially detrimental effects. Many cancers are associated with altered epigenetic profiles, leading to altered expression of the genes involved in cell growth or differentiation. Autoimmune and neoplastic diseases increase in frequency with increasing age, with epigenetic dysregulation proposed as a potential explanation. In support of this hypothesis, studies in monozygotic twins revealed increasing epigenetic differences with age. Differences in methylation status of CpG sites, monoallelic silencing, and other epigenetic regulatory mechanisms have been observed in key inflammatory response genes. The importance of the epigenome in the pathogenesis of common human diseases is likely to be as significant as that of traditional genetic mutations. With advances in technology, our understanding of this area of biology is likely to increase rapidly in the near future.