In this chapter, the IL-2 family of cytokines' is defined to include IL-2, IL-4, IL-7, IL-9, IL-13, IL-15, and thymic stromal lymphopoietin (TSLP). We define the immediate' IL-2 family as IL-2, IL-4, IL-7, IL-9, and IL-15, as these five cytokines share thecommon cytokine receptor chain, c, which was first discovered as a component of the IL-2 receptor. Moreover, IL-2 was the first of these five cytokines to be identified and cloned. IL-13 and TSLP can be considered as slightly more distant relatives as theirreceptors do not share c. As discussed below, IL-13 is particularly closely related to IL-4 whereas TSLP is closely related to IL-7. Details of the basic properties of both human and murine versions of these cytokines, including cellular source, molecular weight,number of amino acids, major actions, chromosomal location, and genomic organization are summarized in Table 1. A number of general properties of four โ-helical bundle `type I' cytokines (which include the IL-2 family cytokines) will be discussed. Then, the bulk of this chapter will focus specifically on the IL-2 family of cytokines, their receptors, the significance of shared receptor components, and aspects of signaling.