The formation of stable cell contacts is of pivotal importance for every metazoan life form. It is therefore not surprising that adhesive molecules appeared early during the evolution of multicellular organisms. The pioneering work of Johannes Holtfreter and others indicated that adhesive molecules, which reside in the plasma membrane on the surface of most cells, are not only important for establishing general cell adhesion and cellular contacts, but also convey a specific tissue and cellular identity to their host cells (1). Over the last few decades a large number of cell adhesion molecules (CAMs) have been identified and further characterized, and we have learned that the expression of these proteins is highly choreographed in terms of timing and cell identity.