Gap junctions are clusters of intercellular channels between adjacent cells. The channels are formed by the direct apposition of oligomeric transmembrane proteins, permitting the direct exchange of ions and small molecules (< 1 kDa) between cells without involvement of the extracellular space. Vertebrate gap junction channels are composed of oligomers of connexins, an enlarging family of proteins consisting of perhaps > 20 members. This article reviews recent advances in understanding the structure of intercellular channels and describes the diverse functions attributable to gap junctions as a result of insights gained from targeted gene disruptions in mice and genetic disease in humans.