Epigenetic regulation of gene expression plays critical roles in differentiation of cells and organs. In mammalian placentation, it is clearly shown that genomic imprinting which is primarily controlled by DNA methylation is essential for placentation. Addition to DNA methylation, histone modifications and non-coding RNAs are also involved in placentation. Recently, it has been shown that epigenetic mutations could cause medical complications during pregnancy such as FGR (Fetal Growth Restriction) and PIH (Pregnancy Induced Hypertension). Developmental epigenetics would contribute to establish new concepts of diseases and provide new treatments in future.