The classification of vasculitis has been an area of controversy for many years, with specific classification criteria only introduced relatively recently. Kussmaul and Maier [1] provided the first description of “periarteritis nodosa” when they described a patient with a systemic illness characterized by numerous nodules along the course of small muscular arteries.

The most widely accepted classification system reflects dominant vessel size and association with antineutrophil cytoplasmic antibodies (ANCA) (Table 2.1). This classification system also broadly reflects the therapeutic approaches that are applied to the different groups. The medium and small vessel group respond well to immunosuppression with cyclophosphamide and corticosteroids, whereas the large vessel group require moderate to high dose steroids and the small vessel group only sometimes require low dose corticosteroids. Also, the small-medium vessel group is the most likely to develop glomerulonephritis and renal failure, is associated with ANCA, and contrast pathogenetically with the small vessel group, which is often associated with immune complexes. This has led to the emergence of the concept of ANCA-associated vasculitis (AAV) which reflects the notion that granulomatosis with polyangiitis (Wegener’s) (GPA), microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (Churg–Strauss) (EGPA) share many features in common and they are often included together in clinical trials.