Eosinophils play important roles in adaptive immune responses, inflammatory processes, and disease states. Recently, considerable research has been devoted to better defining the normal and abnormal biology of these cells, specifically their origin and mechanisms of stimulation, chemotaxis, regulation, and activation. Interleukin-5 has been identified as a major regulator of eosinophil development and function. This review highlights current literature on interleukin-5 and eosinophil production. Areas covered include molecular signaling, physiologic sources of interleukin-5, and interactions between interleukin-5, eosinophils, and the bone marrow microenvironment. Clinical correlates are also presented.