Thymus and activation-regulated chemokine (TARC) was the first CC chemokine identified as a T-cell chemoattractant. It is known as CC chemokine ligand 17 (CCL17) according to the new nomenclature and shares most amino acid identity with CCL22. CCL17 expression is highest in the thymus, but can also be detected in other tissues such as the small intestine, colon, and lung. CCL17 is completely absent from the spleen. Dendritic cells are the predominant source of constitutive CCL17 production, whereas most immune cells and structural cells are capable of inducible expression. CCL17 primarily acts through its receptor CCR4 as a chemoattractant of T-helper-2 (Th2) cells. However, CCR4 is also found on Th1 cells, regulatory T cells, natural killer (NK) T cells, NK cells, platelets, and macrophages. CCL17 inhibits classical macrophage activation and type 1 responses, which further drive type 2 immune responses. CCL17 is associated with several Th2-mediated respiratory diseases such as bronchial asthma, allergic rhinitis, pulmonary fibrosis, and eosinophilic pneumonia, and represents a potential target for treatment of these pathologic disorders.