Genomic imprinting is an epigenetic phenomenon that differentiates maternal and paternal copies of genes in the genome and causes monoallelic expression depending on parental origin. Imprinting is an evolutionary puzzle, as it bears the costs of diploidization without its advantages, namely, protection from recessive mutations. The aim of this review is to answer the question of why genomic imprinting arose and became fixed in the evolution of angiosperms, insects, marsupials, and placental mammals.