Ordered cell-cell and cell-matrix adhesive interactions are a fundamental feature of all multicellular organisms. Numerous studies in recent years have confirmed that both types of cell adhesion are mediated by cell surface receptors known as "adhesion molecules". These receptors can be divided into a limited number of families. Knowledge gained from basic research into cell adhesion is now being applied to clinical problems, and some of these have been summarized here. Integrins are heterodimeric proteins mediating cell-cell and cell-extracellular matrix adhesive connections and signal transduction across the plasma membrane. The important roles of integrins are in Leukocyte Adhesion Deficiency, in viral diseases, neural development and cancer. Suggestive data now points to roles in functions characterized in part by morphological rearrangements, such as learning and memory, and injury responses.