In this monograph, an accurate ultrasonic method for measurement of small motion and deformation of biological tissue are described. In the displacement estimation based on the phase change of echoes, the displacement estimates are biased when the center frequency of the RF echo changes. Such an apparent change in the center frequency could be caused by the interference of echoes from scatterers. To reduce the influence of the center frequency variation on the estimation of motion and deformation, an error correcting function, which does not require the assumption that the center frequency distributions in 2 different frames are the same, was introduced. As a result, the proposed method provides better strain estimates in comparison with conventional phase-sensitive correlation methods.