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 granulocyte-macrophage colony-stimulating factor (GM-CSF) and is predominantly regulated at the transcriptional level. A variety of stimuli and modulators have been identified as regulating 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 crucial for the development of new therapeutic agents for allergic disease. This review covers the major molecular aspects of IL-5 research.