Although much has been learned about the basic contents and capabilities of eosinophils, some of the roles eosinophils play in host defense and the immunopathogenesis of diseases remain enigmatic. In addition to containing four notable cationic granule proteins and their ability to synthesize lipid mediators of inflammation, eosinophils have recently been shown to be able to elaborate a range of cytokines that may exert autocrine as well as paracrine effects. The roles of eosinophils within tissues are modulated by interactions with the extracellular matrix and other cells during eosinophil recruitment and activation, and eosinophils may engage in cooperative interactions with other cells.