The systemic vasculitides are rare inflammatory conditions of blood vessel walls. A number of different classification schemes have been published since the first in 1952. The important developments have been the recognition of dominant blood vessel size, the distinction between primary and secondary vasculitis and the incorporation of pathogenic markers such as anti-neutrophil cytoplasmic antibodies. In 1990 the American College of Rheumatology (ACR) published criteria for the diagnosis of polyarteritis nodosa, Churg-Strauss syndrome, Wegener's granulomatosis, hypersensitivity vasculitis, Schönlein-Henoch purpura, giant cell arteritis and Takayasu arteritis. Sensitivity and specificity rates varied considerably: 71.0-95.3% for sensitivity and 78.7-99.7% for specificity. The criteria were not tested against the general population or against patients with other connective tissue diseases or rheumatic conditions. Four years later the Chapel Hill Consensus Conference (CHCC) produced definitions for the major types of vasculitis, however, these have proved controversial. Comparison in unselected patients with systemic vasculitis (in particular polyarteritis nodosa and microscopic polyangiitis) has shown that the ACR criteria and CHCC definitions identify different patients. The systemic vasculitides are somewhat more common than previously believed. The overall annual incidence approaches 40/million adults. The most common form of primary systemic vasculitis is giant cell arteritis; Wegener's granulomatosis, microscopic polyangiitis and Churg-Strauss syndrome have similar incidences. Classical polyarteritis nodosa and Takayasu arteritis are very rare in the UK.