The systemic vasculitides are a group of multisystem diseases characterized by inflammation of blood vessels. The aetiopathogenesis is unknown, and therefore nomenclature and classification are often descriptive and based on pathological features. Generally agreed classification schemes are vital to enable large multicentre or multinational clinical trials to be undertaken. An algorithm has recently been developed to harmonize use of the American College of Rheumatology (ACR) 1990 criteria and the Chapel Hill Consensus Conference definitions. Despite this, a revision of the classification criteria is still needed, and diagnostic criteria need to be developed ab initio. The very complexity of the diseases makes accurate objective assessment critical, especially for the conduct of clinical trials. Several standardized assessment tools for both disease activity and damage have been developed over the past two decades and are now widely used in both clinical trials and routine practice. A second generation of tools is now under development.