Investigations are being conducted to develop a method using motion induced by radiation force to monitor HIFU lesions, by virtue of their increased stiffness. A therapeutic transducer, periodically excited at subthreshold levels, generates the radiation force: a collinear diagnostic transducer monitors the degree and time-course of induced motion. Lesions are detected by changes in pre- and post-treatment motion patterns. In vitro experiments and computer simulations have been performed to clarify the roles of several phenomena so that optimized systems can be designed for practical on-line applications. Lesion attenuation has been found to be a key factor since it directly affects radiation force. Lesions with near-normal attenuation exhibit less motion than normal, because of increased stiffness. Lesions with significantly higher attenuation can exhibit increased motion, due to increased force, but they demonstrate spatial patterns differing from normal. The profile of the push beam was found to affect the magnitude, spatial pattern, and time course of induced motion. The diagnostic transducer’s bandwidth and beam profile were shown to affect the precision and sensitivity of motion characterization. Quantitative findings from these studies are being combined with thermal studies to design monitoring systems for specific applications.