Disease activity in rheumatoid arthritis is not easily measured. The validity of current measures has been reviewed. Recent international efforts have resulted in consensus over a minimum (WHO/ILAR) core set of endpoints in RA trials: pain, patient and physician (assessor) global assessment, physical disability, swollen joint count, tender joint count, acute phase reactants; and for studies of one or more years' duration: radiographs of joints. The near future will hopefully see validation of new measures, also in terrains not currently covered by the core set.