The Disease Activity Score (DAS), its modified version the DAS28, and the DAS-based European League Against Rheumatism (EULAR) response criteria are well-known measures of disease activity in rheumatoid arthritis (RA). The DAS is a clinical index of RA disease activity that combines information from swollen joints, tender joints, the acute phase response, and general health. The EULAR response criteria classify individual patients as non-, moderate, or good responders, depending on the extent of change and the level of disease activity reached. The DAS, DAS28, and EULAR response criteria have been validated extensively. For daily practice, it has been shown that a tight control strategy, including measurement of disease activity using the DAS and planned adjustment of antirheumatic medication, is an effective strategy for RA. This article summarizes the development and validation of the DAS and DAS28 and their use in clinical trials and practice for the clinician.