Detecting pathology using the "stiffness" of the tissue is more that 2000 years old. Even today it is common for surgeons to feel lesions during surgery that have been missed by advanced imaging methods. Palpation is subjective and limited to individual experience and to the accessibility of the tissue region to touch. It appears that a means of noninvasively imaging elastic modulus (the ratio of applied stress to strain) may be useful to distinguish tissues and pathologic processes based on mechanical properties such as elastic modulus. The approaches to date have been to use conventional imaging methods to measure the mechanical response of tissue to mechanical stress. Static, quasi-static or cyclic stresses have been applied. The resulting strains have been measured using ultrasound or MRI and the related elastic modulus has been computed from viscoelastic models of tissue mechanics. Recently we have developed a new ultrasound technique that produces speckle free images related to both tissue stiffness and reflectivity. This method, termed "Ultrasound Stimulated Vibro-acoustography" (Science 280:82-85, April 3, 1998; Proc Natl Acad Sci USA 96:6603-6608, June 1999), uses ultrasound radiation pressure to produce sound vibrations from a small region of the tissue that depend in part on the elastic characteristics of the tissue. The method can detect micro-calcification within breasts, and promises to provide high quality images of calcification within arteries. In addition, vibro-acoustography can detect mechanical defects in certain prostheses such as artificial mitral and aortic valves. Extensions of the method include vibrometry, in which motion of an object is detected with laser vibrometry or an accelerometer, and shear wave detection, in which the resulting shear waves within objects such as arteries or tissue are detected with Doppler or MRI. Keywords : vibrometry, stiffness, ultrasound, acoustic, shear waves