Interleukin 5 (IL-5) is a cytokine primarily involved in the pathogenesis of atopic diseases. It specifically controls the production, activation, and localization of eosinophils, the major cause of tissue damage in atopic diseases. IL-5 belongs to a gene family shared by IL-3, IL-4, and GM-CSF and is predominantly regulated at the transcriptional level. The homo-dimeric IL-5 protein is well conserved between species and primarily produced by T cells but also in low-levels by mast cells, В cells, and eosinophils. A variety of stimuli and modulators have been identified to regulate production of IL-5 both in vivo and in vitro, indicating a highly complex series of control mechanisms. However, a better understanding of the biology of IL-5 and the regulation of its expression is critical for development of new therapeutic agents for allergic disease. This chapter covers the major biological, molecular, and structural aspects of IL-5 research.