Developmental events in preimplantation mouse embryos include the first cleavage, the activation of the embryonic genome, the compaction of the blastomeres to form morula (MO), and the formation of the blastocyst (BL). Compaction, the first cell differentiation event in mammalian development, occurs at the late eight-cell stage in the mouse and may be described in terms of some types of morphological change, which involve reorganization within a cell and intercellular reorganization. Surface microvilli became restricted to a few basal sites and to an externally facing (apical) pole. Prior to compaction, the blastomeres are spherical and lack specialized intercellular junctions. During compaction, the cells were flattened against one another, thus maximizing intercellular contact and obscuring intercellular boundaries. It is believed that the events of compaction have an important influence on the processes involved in blastocyst formation, namely the initiation of inner cell mass and trophectoderm differentiation. The inner cell mass will form the future embryo proper, whereas the trophectoderm cells will form only extraembryonic tissues. Compaction is initiated by E-cadherin mediated cell adhesion, which is regulated post-translationally via protein kinase C. With E-cadherin knock-out, maternal E-cadherin is able to mediate the compaction process at the morula stage. Initial adhesion is mediated by homophilic interactions between E-cadherin extracellular domains.In this review, we attempted to describe this process in detail.