Leukocyte homing is a complex, multistep process involving the coordinated expression of adhesion molecules and certain chemotactic cytokines, termed chemokines. Although chemokines initially burst into the literature as potent inflammatory mediators, it is now clear that they are involved in a variety of processes including lymphocyte maturation, angiogenesis, and tumor growth. Furthermore, a variety of important pathogens manipulate various chemokine/receptor pathways to infect the host and evade the immune system.