Breast and prostate tumors or cancers tend to be stiffer than the surrounding normal tissue. However, the difference in echogenicity between cancerous and normal tissues is not clearly distinguishable in ultrasound B-mode imaging. Thus, imaging the stiffness contrast between the two different tissue types helps to diagnose lesions quantitatively, and such a method of imaging the elasticity of human tissue is termed ultrasound elasticity imaging. Recently, elasticity imaging has become an effective complementary diagnostic modality along with ultrasound B-mode imaging. This paper presents various elasticity imaging methods that have been reported up to now and describes their characteristics and principles of operation.