Genomic imprinting is a system of non-Mendelian inheritance that is unique to mammals. Two types of imprinted genes show parent-of-origin-specific expression patterns: the paternally expressed genes (Pegs), and the maternally expressed genes (Megs). Parental genomic imprinting memory is maintained in the somatic cell lineage and regulates the expression of Pegs and Megs, while it is erased and re-established in the germ cell lineage according to the sex of the individual. The paternal and maternal imprinting mechanisms, which regulate different sets of Pegs and Megs, are essential for establishing the parental expression profiles of imprinted genes that are observed in sperms and eggs. Based on recent evidence, we outline the relationship between parental imprinting and the expression profiles of Pegs and Megs and discuss a novel view of the regulation of genomic imprinting. We also discuss the biological significance of genomic imprinting and propose hypotheses on the essential nature of genomic imprinting and the close relationship between genomic imprinting and the acquisition of placental tissues during mammalian evolution.