Integrins are a structurally elaborate family of adhesion molecules that transmit signals bidirectionally across the plasma membrane by undergoing large-scale structural rearrangements. By regulating cell-cell and cell-matrix contacts, integrins participate in a wide-range of biological interactions including development, tissue repair, angiogenesis, inflammation and hemostasis. From a therapeutic standpoint, integrins are probably the most important class of cell adhesion receptors. Structural investigations on integrin-ligand interactions reveal remarkable features in molecular detail. These details include the atomic basis for divalent cation-dependent ligand binding and how conformational signals are propagated long distances from one domain to another between the cytoplasm and the extracellular ligand binding site that regulate affinity for ligand, and conversely, cytosolic signaling pathways.