Acoustic radiation force is a phenomenon associated with the propagation of acoustic waves in attenuating media. Attenuation includes both scattering and absorption of the acoustic wave. Attenuation is a frequency dependent phenomenon, and in soft tissues it is dominated by absorption. With increasing acoustic frequencies, the tissue does not respond fast enough to the transitions between positive and negative pressures, thus its motion becomes out of phase with the acoustic wave, and energy is deposited into the tissue. This energy results in both a net force in the direction of wave propagation, and tissue heating. The force causes a displacement of the tissue, and the time scale of this response is much slower than that of the acoustic propagation. This interaction of sound with tissue can be used to derive additional information about the tissue, beyond what is normally provided in an ultrasonic image. The magnitude, location, spatial extent, and duration of acoustic radiation force can be controlled to interrogate the mechanical properties of the tissue. Our laboratory studies this phenomenon, and is developing novel methods for utilizing acoustic radiation force to generate images of tissue stiffness. We implement these methods on a commercially available diagnostic ultrasound scanner, thus translation and implementation in the clinic is readily accomplished.