ANCA-associated vasculitis (AAV) is a rare and potentially fatal autoimmune disorder characterized by pauci-immune necrotizing vasculitis affecting small- to medium-sized blood vessels. The pathogenic role of ANCAs in AAV is supported by both clinical and experimental evidence, and when used in the proper clinical setting, ANCA testing is highly specific for AAV. Testing with both indirect immunofluorescence assay and enzyme immunoassay may increase sensitivity for AAV; however, testing with a high-quality enzyme immunoassay may be used alone. Nonvasculitic conditions and drugs can cause ANCA positivity without manifestations of AAV. We review ANCA testing itself, performance characteristics and specific conditions for the laboratory test, and various conditions when ANCA testing is useful in diagnosis, disease monitoring, and selecting treatment.