Mammalian embryo implants at different and species-specific stages, so that the length of preimplantation phase greatly varies with species. The phase common to all species extends from fertilization to formation of a blastocyst with three specified lineages : epiblast and two extraembryonic ones trophectoderm and primitive endoderm (also called hypoblast). During this period, the embryo goes from a maternal to an embryonic control of its development. This transition relies on both the progressive transcriptional activation of the embryonic genome and the use and degradation of maternal information. Building a totipotent zygotic genome from two genomes of highly differentiated gametes implies extensive reprograming of the embryonic epigenome. Recent data evidenced specific features of chromatin organization in early embryo, correlated to a weak and promiscuous gene expression. This progressively evolves towards a more canonical chromatin organization and a precise regulation of gene expression. Then, first lineages specification relies on fine tuning of gene expression. The mechanisms responsible for this are well known in the mouse model but may vary with species.