Certain organs, because of their finite thickness with respect to the shear wavelength, produce guided wave behavior. This chapter summarizes experimental results of in vivo application of guided waves in the viscoelastic assessment of the myocardium, large arteries, bladder, cornea, and tendons. The Lamb wave dispersion ultrasound vibrometry method allows for the estimation of elasticity and viscosity of the myocardial wall throughout the course of the cardiac cycle. Bernal studied arterial elasticity by identifying the modes of wave propagation in tubes made out of urethane rubber and excised pig carotid arteries using the two-dimensional fast Fourier transform. The ex vivo experiments showed that ultrasound bladder vibrometry (UBV) can track changes of the bladder wall's elastic properties, moreover there was a high correlation between the pressure measurements and the shear modulus estimated using UBV.