Genetical as well as experimental embryology methods have permitted to uncover a very important feature of mammalian embryonic development: it has been shown that female and male genomic complements are differentially imprinted in such a way that contribution of both a maternally and a paternally derived genome are absolutely necessary for the embryo to complete its normal development. The paternal and maternal genomes are not equivalent and have a complementary role during development in mammals. The differences in activity of each parental genome result from an epigenetic modification of the genome during gametogenesis: the parental imprinting. The recent discovery of several mouse and human genes which are imprinted should permit to address new data of some diseases.