Leukocyte recruitment from the circulation into inflammatory tissues requires a series of soluble and cell-bound signals between the responding leukocyte and vascular endothelial barrier. Chemotactic factors are believed to be responsible for this selective adhesion and transmigration. A superfamily of small, soluble, structurally-related molecules called 'chemokines' have been identified and shown to selectively promote the rapid adhesion and chemotaxis of a variety of leukocyte subtypes both in vivo and in vitro. Chemokines are produced by almost every cell type in the body in response to a number of inflammatory signals, in particular those which activate leukocyte-endothelial cell interactions. These molecules also appear to play important roles in hematopoesis, cellular activation, and leukocyte effector functions. In addition, chemokines have been found in the tissues of a variety of disease states characterized by distinct leukocytic infiltrates, including rheumatoid arthritis, sepsis, atherosclerosis, asthma, psoriasis, ischemia/reperfusion injury, HIV replication, and a variety of pulmonary disease states. This review will primarily focus on the role of chemokines in cell adhesion and trafficking as well as their role as effector molecules.