The understanding of genomic imprinting has made us realize that maternal and paternal contributions to the embryo are different. Disturbances during the imprinting process may lead to different pathologies due to an imbalance of gene expression either maternally or paternally derived. Known epigenetic diseases such as neonatal diabetes, growth retardation or overgrowth syndromes as well as cancer are better understood. It has become clear that environmental factors can be at the origin of such epigenetic changes. Careful analysis and diagnosis of epigenetic diseases are important for patient treatment and outcome.