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The mechanical attributes of soft tissues depend on their molecular building blocks (fat, collagen etc.), on the microscopic and macroscopic structural organization of these blocks (Fung 1981), and on the boundary conditions involved. These mechanical attributes may include the shear or elastic moduli (Young’s modulus), the Poisson’s ratio, or any of the longitudinal or shear strains that occur in tissues as a response to an applied load. In the normal breast, for example, the glandular structure may be firmer than the surrounding fibrous connective tissue, which in turn is firmer than the subcutaneous adipose tissue. Pathological changes are generally correlated with changes in tissue stiffness as well. Many cancers, such as scirrhous carcinomas of the breast, seem much stiffer and less mobile than benign (fibroadenoma) tumors (Anderson 1953). In many cases, in spite of the difference in stiffness or mobility, the …