Systemic vasculitides have been classified according to several systems, all of which derive from the system originally proposed by Zeek in 1953. In 1990 the American College of Rheumatology published consensus definitions for the various forms of systemic vasculitis known at that time. Over the next few years it became clear that serologic tests for various anticytoplasmic neutrophil antibodies (ANCAs) would be very helpful in the clear separation of these disorders, and so in 1994 the American College of Rheumatology issued revised consensus criteria taking the new data into account. In this system, which is most widely used currently, the disorders are classified according to the size and type of vessel affected, the pattern of organ dysfunction, and various serologic markers. The histologic features are also regarded as an important element, but the authors stress that a correct diagnosis cannot be assured by biopsy findings alone.