Epigenetics is defined as the study of changes in gene function that are mitotically and/or meiotically heritable and that dp not entail a change in DNA sequence. Epigenetic regulation of gene expression plays a critical role in development and differentiation, X inactivation, genomic imprinting and several human diseases, including cancer. DNA methylation and histone modifications play an essential role in epigenetic gene expression. In this article, an outline of epigenetics and representative analytic methods are explained. Then, epigenetic alterations and clinical implications of diseases, such as cancer, genomic imprinting-related diseases, and others, are described. The great advance in epigenetic research, from basic to clinical, will bring significant benefits to human health and a broad range of life sciences.