Antineutrophil cytoplasmic antibodies (ANCA) are a heterogeneous group of autoantibodies with a wide and diverse range of clinical associations. In vasculitis, the diagnostic utility of proteinase 3 (PR3)-ANCA and myeloperoxidase-ANCA for Wegener's granulomatosis and microscopic polyangiitis, respectively, is now well established. Because of their significance as tools for diagnosis and prognosis, these autoantibodies have been analyzed extensively as markers for underlying immunopathogenic disturbances. In this review, we consider recent advances in the understanding of ANCA, focusing on their detection, diagnostic value, and role in the pathogenesis of vasculitis. In addition, promising new ways have been developed to elucidate the pathophysiologic and diagnostic relevance of the ANCA target antigens PR3 and myeloperoxidase. A great deal of attention and controversy has focused on the possible mechanisms underlying the ANCA-related immune response, such as antigenic cross-reactivity between human polymorphonuclear leukocyte proteins and extrinsic antigens by molecular mimicry, idiotype network regulation, and T cell reactivity to PR3 and myeloperoxidase.