Chemokines are chemotactic cytokines that control the migration and positioning of immune cells in tissues. The chemokine universe is comprised of approximately 50 peptides and 20 receptors in humans. The chemokine ligand super-family is further partitioned into subgroups of the largest (CC chemokines; 28 members) and second largest (CXC chemokines; 16 members) families. The action of chemokines toward responsive cells is gradient-dependent chemoattraction in vitro, assigning chemokines a role in leukocyte trafficking. Several chemokine-receptor pairs are exclusive; for other chemokine receptors, responses can be elicited by as many as 10 individual ligands. Some chemokines can productively signal to as many as three receptors.