Eosinophils are normal cellular components within mucosal tissues, as well as being participants in varied allergic, immunological, and other disease processes (1). Increases in blood eosinophilia may accompany these diseases, but it is in tissues, normal mucosal tissues of the gastrointestinal and respiratory tracts, as well as mucosal and other tissue sites in association with specific disease processes, that eosinophils characteristically are found. It is in these normal and diseased tissues that eosinophils function and contribute to the immunopathogenesis of various diseases (2). The functions of eosinophils may be complex. On the one hand, eosinophils are capable of acute cellular responses. These acute responses include degranulation, oxidative burst activity, and the elaboration of lipid mediators, notably leukotriene C4(LTC4) and platelet-activating factor (PAF). On the other hand, eosinophils can interact collaboratively with other cellular elements. Eosinophils may respond to cytokines elaborated by other cell types. Some of these cytokines, such as GM-CSF, IL-3, and IL-5, prolong eosinophil viability and modulate the acute responses of eosinophils. Other cytokines, including some lymphokines, stimulate eosinophils without heightening acute cellular responses. Moreover, eosinophils themselves may both elaborate cytokines and provide other mechanisms for stimulating adjacent cells. In this chapter, we will review studies related to both aspects of eosinophil functioning. First, we 282will consider some of the aspects pertinent to the intracellular sites and regulation of eosinophil eicosanoid formation. Second, we will consider findings pertinent to intercellular interactions between lymphocytes and eosinophils.