The vasculitides are a heterogeneous group of diseases that are characterized by blood vessel inflammation and necrosis. They have a wide spectrum of manifestations due to the involvement of arteries and other vessels of various sizes and locations. Classification criteria are useful in improving our understanding of the epidemiology of these conditions but they are not diagnostic criteria. In recent years a progressive increase in incidence has been reported. Both giant cell arteritis and Wegener's granulomatosis are more common in the Northern hemisphere. Environmental factors have been implicated in the pathogenesis of these syndromes. Recent studies in patients with large and small-sized vasculitides support a genetic influence in disease susceptibility. They have confirmed the association of giant cell arteritis and Henoch-Schönlein purpura with HLA-DRB1 alleles. Moreover, the polymorphisms of other genes, such as interleukin-1 receptor antagonist, seem to be implicated in disease severity in patients with cutaneous vasculitis.