Reproductive isolation is considered to play a key part in evolution, and plants and animals have developed a range of strategies that minimize gene flow between species. In plants, these strategies involve either pre-zygotic barriers, such as differences in floral structure and pollen-stigma recognition, or post-zygotic barriers, which are less well understood and affect aspects of seed development ranging from fertilization to maturation. In most angiosperms, a double fertilization event gives rise to a zygote and the endosperm: a triploid tissue with an unequal parental genomic contribution, which, like the placenta of mammals, provides reserves to the developing embryo. Interestingly, many aspects of endosperm development, again like the placenta, are regulated by a range of epigenetic mechanisms that are globally termed imprinting. Imprinted genes are characterized by their uniparental expression, the other parental allele being silenced. Normal development of the endosperm thus requires a highly specific balance of gene expression, from either the maternal or paternal genomes. Any alteration of this balance resulting from changes in allelic copy number, sequence or epigenetic imprints can cause endosperm failure and eventual seed abortion. In its widest sense, the endosperm thus serves as an accurate 'sensor' of compatibility between parents. A first step in understanding this important, yet complex system must clearly be the isolation and characterization of as wide a range as possible of imprinted genes.