The integrins are receptors for proteins of the extracellular matrix, both providing a physical link to the cytoskeleton and transducing signals from the extracellular matrix. Activation of integrins leads to tyrosine and serine phosphorylation of a number of proteins, elevation of cytosolic calcium levels, cytoplasmic alkalinization, changes in phospholipid metabolism and, ultimately, changes in gene expression. The recently discovered focal adhesion kinase localizes to focal contacts, which are sites of integrin clustering, and focal adhesion kinase can physically associate with integrins in vitro. As integrins lack intrinsic catalytic activity, focal adhesion kinase is a candidate for a signaling molecule that is recruited by integrins in order to trigger the generation of intracellular second messengers. Thus, focal adhesion kinase may play a central role in signal transduction through integrins.