Quantitative measurement has led to major advances in the diagnosis, prognosis and management of chronic diseases. Quantitative measures in rheumatic diseases differ from measures in many chronic diseases in several respects. There is no single "gold standard," such as blood pressure or cholesterol, in the diagnosis, management, and prognosis of any rheumatic disease. Laboratory tests are limited; for example, in rheumatoid arthritis > 40% of patients or more have a normal erythrocyte sedimentation rate (ESR). Formal joint counts have poor reliability and are not performed at most visits of most patients. Radiographs are rarely read quantitatively, except in formal clinical trials. The optimal quantitative measures to monitor status and assess long-term prognosis are often derived from patient self-report questionnaires. Quantitative measures may reflect disease activity, e.g., swollen joint counts or C-reactive protein (CRP), long-term damage, e.g., radiographic damage, or poor outcomes, e.g., work disability and premature death. Disease activity measures used in clinical trials are primarily surrogates for long-term outcomes. As there is no single "gold standard" measure, indices of multiple measures are used in patient assessment. Indices used in rheumatoid arthritis assess primarily disease activity, but separate indices have been developed to assess disease activity versus damage in patients with ankylosing spondylitis, systemic lupus erythematosus, and vasculitis.