This paper summarizes dynamic measurements of shear modulus constants acquired for spontaneously growing rat mammary tumors. Measurements are compared with histology to determine tumor types. We also report on 3D shear-wave velocity fields acquired from an inhomogeneous hydrogel phantom with known mechanical properties. Phantom measurements enable us to interpret the effects of tissue structure and geometry on viscoelastic parameter estimates, and demonstrate that reproducible measurements are possible in vivo. Viscoelastic properties describe the mechano-environment of cells undergoing malignant transformation and tumor growth.