Maternally and paternally derived chromosomes might be expected to contribute equally to the various cellular and developmental processes in placental mammals and flowering plants. However, this is not true even in the case of the self-pollinated plant, Arabidopsis, which has identical DNA sequences in both parental genomes. The reason for this is that some genes, called "imprinted genes", are expressed exclusively from paternally or maternally inherited chromosomes. As a result, parental chromosomes express a distinct set of genes and play different roles in biological processes. Here, we review and compare roles of genomic imprinting in flowering plants and placental mammals. © 2008 Elsevier Ltd. All rights reserved.