What do cell adhesion, cell migration during embryogenesis, thrombosis, and lymphocyte help and killing have in common? It has recently become clear that these ostensibly unrelated processes all involve related cell surface receptors. This realization comes from sequence data, which show that cell surface receptors for extracellular matrix proteins, a major glycoprotein complex on platelet cell surfaces, and two groups of glycoprotein antigens on lymphoid and myeloid cells all contain homologous subunits. In this brief review, I summarize data from these diverse fields and attempt to clarify the relationships among the members of this family of cell surface receptors.