This chapter explores what imprinted expression is, and the epigenetic mechanisms that control it. Genomic imprinting is regulated by epigenetic mechanisms. The acquisition of post-fertilization differential methylation requires the gametic imprinting control regions (ICR) and is thought to reinforce imprinted gene expression. Two episodes of genome-wide epigenetic reprogramming that are required for viability and reproductive success occur during early life. In mammals the evolution of imprinting coincides with the evolution of placentation. Many imprinted genes play a direct role in fetal growth in addition to their role in placental development, which complicates the interpretation of genetic models. Imprinted genes play critical roles in the development of key metabolic organs, with obvious consequences for postnatal metabolism. The monoallelic restriction of imprinted gene expression is fundamentally dependent on epigenetic modifications that allow the transcriptional machinery to distinguish between the two parentally inherited chromosomes.