Gap junctions are unique intercellular channels that connect the cytoplasm of adjacent cells. They are the only channels that mediate the direct cytoplasmic exchange of small hydrophilic molecules between cells--a process called gap junctional communication. Formed by a family of integral membrane proteins called connexins, gap junctions are dynamic multifunctional complexes that are essential for healthy vertebrate development and physiology. Defects in connexin proteins, and, therefore, in gap junctional communication, are associated with a large variety of pathologies in humans and experimental animals. Thus, knowledge of the molecules that pass through gap junction channels is extremely important. However, aside from some notable cases, the repertoire of biologically important transjunctional molecules remains relatively unexplored. Indeed, the study of the intercellular transfer of endogenous molecules presents formidable challenges. Here we review developments in identifying biologically relevant molecules that pass between cells through gap junction channels.