Chemokines are small peptides that are potent activators and chemoattractants for leukocyte subpopulations and some nonhematopoietic cells. Based on genomics efforts, it has been estimated that there may be as many as 40 to 50 human chemokines. The actions of chemokines are mediated by a family of 7-transmembrane G-protein-coupled receptors, the size of which has grown considerably in recent years and now includes 18 members. Lately, it has become clear that chemokines are involved in a wide variety of functions in normal and disordered physiology.