The Churg-Strauss syndrome is part of the necrotizing vasculitis group of diseases. Its clinical diagnosis rests on the occurrence of a severe asthma followed, more or less closely, by systemic manifestations that are predominantly neurological, cutaneous, gastrointestinal and cardiovascular. To these must be added pulmonary infiltrates, eosinophilia and increase in serum IgE's. Three histological criteria (necrotizing vasculitis of the small vessels, perivascular infiltration with numerous eosinophils and extravascular granulomas) enable this anatomico-clinical syndrome to be identified, but the granuloma component is frequently missing and the existence of borderline states, notably with periarteritis nodosa, is unquestionable. Corticosteroids and immunodepressants have transformed the prognosis of the Churg-Strauss syndrome, while its physiopathological mechanism remains mysterious.