Many cell-cell and cell-matrix interactions depend upon the engagement of specific ligands by members of the integrin family of cell-adhesion receptors. In concert with the identification of new integrins, the number of integrin ligands continues to expand dramatically. The diversity of the integrin ligands bridges many areas of cell and molecular biology. Ligand recognition by integrins requires not only the presence of the cognate primary sequence within an appropriate secondary structure, but also the correct tertiary and quaternary structure of the ligand. Presentation of an 'activated' ligand sequence to specific contact sites within the integrin under specified divalent-cation conditions is necessary for a productive and high-affinity interaction.