In 1951 the pathologists Churg and Strauss identified 13 patients who presented with a clinical syndrome characterised by asthma, hypereosinophilia, and evidence of vasculitis affecting a number of organs.1 The three main histological features found on pathological examination of these cases were extravascular granulomas, tissue eosinophilia, and necrotising vasculitis (fig 1). Most of these patients had been previously diagnosed with “periarteritis nodosa”. After reviewing a number of cases of periarteritis nodosa without asthma and finding no evidence of an eosinophilic granulomatous process, Churg and Strauss suggested that their 13 cases represented a separate disease process and coined the term “allergic granulomatosis and angiitis”. Later “Churg-Strauss syndrome” (CSS) became the accepted title of this distinctive form of systemic vasculitis. More recent pathological case series involving patients with CSS have highlighted the fact that not all patients have the three main histological features originally described by Churg and Strauss.2 ,3 Given the absence of a histological definition, it was proposed by Lanham et althat the following clinical criteria were required for the diagnosis of CSS: asthma, peripheral blood eosinophilia >1.5 × 10⁹/l (or >10% of total white cell count), and evidence of a systemic vasculitis involving two or more extrapulmonary organs.2 More recently the American College of Rheumatology has developed diagnostic criteria for CSS when there is biopsy proven vasculitis.4 In this review we provide a clinical overview of CSS, review the evidence on which treatment protocols are based, and discuss the current understanding of the pathogenesis of the disease. In addition, we highlight recent controversies in the classification of systemic vasculitis and possible alternative management strategies.