Adhesion is a crucial requirement for the correct regulation of immune and inflammatory responses. In the immune system, leukocytes can interact with each other and with vascular endothelium as well as with extracellular matrix components, changing rapidly and transiently from circulating non-adherent to adherent states. Most of these interactions are mediated by integrins. This review will focus mainly on the structure and function of integrins expressed by leukocytes. The mechanisms for regulating the functional activity of these adhesion receptors, as well as the intracellular signals transduced through integrins, are described.