Eosinophils have been studied using various models. Resulting data indicate that these polynuclear cells may have good and bad effects on the host. Eosinophils have been shown to destroy parasites in vitro but also to contribute to inflammation especially in association with bronchial asthma. These findings illustrate the functional versatility of a cell with the ability to interpret messages from its environment including chemokines, cytokines, growth factors, lipid mediators, and neuropeptides. In succession, these signals can stimulate eosinophils to express a range of regulated (homeostasis) or unregulated (pathogenesis) activity. Some signals cause selective migration into target tissues, especially submucosa in contact with the environment. Other signals determine local function. Understanding these mechanisms will be crucial to development of effective management of persistent hypereosinophilia syndromes.