Acoustic radiation force imaging methods utilize acoustic radiation force to mechanically excite tissue, and the tissue response is monitored with conventional imaging methods. The data provide information about the stiffness of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one such method that is implemented on a modified diagnostic ultrasound scanner, using the same transducer for both the excitation and for monitoring the tissue response. ARFI images are generated of the tissue displacement magnitude within the region of excitation at a given time after radiation force application. ARFI images portray relative differences in tissue stiffness and provide interesting structural information that is well correlated with matched B-mode images. In many instances, ARFI images demonstrate improved contrast over conventional ultrasound images. This imaging mode is under investigation for many clinical applications including: breast, liver, and colorectal imaging, and monitoring RF-ablation procedures. This chapter discusses some of the fundamental concepts associated with ARFI imaging.