Measurement of radiographic abnormalities in metric units has been reported by several investigators during the last 15 years. Measurement of joint space in large joints has been employed in a few trials to evaluate therapy in osteoarthritis. Measurement of joint space width in small joints has been reported by several investigators but has not yet found a place in clinical trials in rheumatoid arthritis or osteoarthritis. We review methods for measuring joint space width in finger, toe, and wrist joints; special attention is given to how the joint edges are found, the method used to measure distance between joint margins, size of an area of the sampled joint, and reproducibility of measurements. Methods for measurement of erosion size, which have had less attention, are briefly discussed.