Eosinophils normally account for only one to three percent of peripheral-blood leukocytes, and the upper limit of the normal range is 450 cells per cubic millimeter of blood. Eosinophilia occurs in a variety of disorders. The most common cause of eosinophilia worldwide is helminthic infections, and the most common cause in industrialized nations is atopic disease. The diverse causes, clinical signs and symptoms, the diagnostic work-up as well as the therapeutic strategies will be discussed in this review. The "hypereosinophilic syndrome" constitutes an evolving concept and is most likely the result of a clonal (neoplastic) disorder. A number of new therapeutic approaches for these conditions have been developed and will be discussed in this article.