Most genes are expressed from both parental chromosomes; however, a small number of genes in mammals are imprinted and expressed in a parent-of-origin specific manner. These imprinted genes play an important role in embryonic and extraembryonic growth and development, as well as in a variety of processes after birth. Many imprinted genes are clustered in the genome with the establishment and maintenance of imprinted gene expression governed by complex epigenetic mechanisms. Dysregulation of these epigenetic mechanisms as well as genomic mutations at imprinted gene clusters can lead to human disease.