Gap junctions are sites where intercellular membrane channels are clustered that allow neighboring cells to pass small molecules directly between them. Gap junctional intercellular communication has been implicated in a variety of human diseases. Gap junction channels are assembled from a large family of proteins called connexins with each type of channel having some unique properties. Preimplantation mouse and rat embryos express multiple connexins and thus potentially contain many types of gap junction channels. Based on experiments focussing on connexin43, gap junction assembly in the mouse begins during compaction in the 8-cell stage and is post-translationally regulated. Gene targeting has been used to create mice lacking individual connexins that are expressed in preimplantation embryos, but none of these experiments has yet revealed a necessary role for any single connexin before implantation. Experiments with anti-connexin antibodies and pharmacological blockers of gap junctional coupling have provided conflicting evidence as to the importance of gap junctions for preimplantation development. However, connexin knockouts have revealed important roles for gap junctional coupling in early postimplantation development. It is proposed that expression of multiple connexins in the blastocyst could prepare the implanting conceptus for rapid diversification of cell types during gastrulation and development of the placenta.