Ultrasound imaging has been used for decades for medical and industrial imaging. Ultrasound in the range of 1-10 MHz has also been used for evaluating tissue properties characterization for many years. Recently, use of low frequency (audio range) vibration for medical diagnosis and evaluation of tissue properties has attracted increased attention. It has been shown that such vibration can reveal important information about tissue mechanical properties that are related to tissue pathology. We use the radiation force of ultrasound to vibrate tissue at low (kHz) frequency range, and record the resulting acoustic response to produce images that are related to the stiffness of the tissue. This method is tested on human breast, liver, heart valve, and arteries. Results show that small microcalcifications can be detected in human breast, calcium buildups can be seen in arteries, and mass lesions can be detected in liver tissues. In these tests, the vibration frequency ranged from 5 to 50 kHz. Another application of the radiation force method is studying solid structures through modal analysis. This method is used to induce vibration in bones and measure their resonance frequencies. The above experimental results suggest that the radiation force method may be a clinically useful tool for detection of pathology in soft tissue and for bone evaluation.