While Interleukin-5 (IL-5) is initially identified by its ability to support the growth and differentiation of activated B cells, overexpression of IL-5 significantly increases eosinophil numbers and antibody levels predominantly from an expanded population of B-1 cells in vivo. Conversely, mice lacking a functional gene for IL-5 or IL-5 receptor alpha chain (IL-5Ralpha) display a number of developmental and functional impairments in B cell and eosinophil lineages. In addition to the JAK-STAT and Btk pathway, the Ras-extracellular signal-regulated kinase (ERK) signals are important for IL-5-dependent cell survival. IL-5 critically regulates expression of genes involved in cell survival, IgH switch recombination, maturation in B cells and genes required for growth, survival, and effector function of eosinophils. IL-5Ralpha expression in B cells, but not in eosinophils is regulated by Oct-2. Eosinophilia is associated with a wide variety of conditions, including asthma and atopic diseases, helminth infections, drug hypersensitivity, and neoplastic disorders. In humans, the biologic effects of IL-5 are best characterized for eosinophils. The Sprouty-related Ena/VASP homology 1-domain containing protein (Spred)-1 negatively controls eosinophil numbers and functions by modulating IL-5 signaling in allergic asthma. We will emphasize that IL-5 plays a pivotal role in the innate and acquired immune response and eosinophilia.