Ultrasonic wave propagation in prestressed materials has been studied extensively in the last 40 years. Most of this work was concentrated on the effect of stress on the velocities of different types of ultrasonic waves in homogeneous materials. Actually stresses affect not only wave velocities but also the boundary conditions at the interface. Many practical applications of ultrasonic stress characterization involve wave propagation through the interface between fluid and solid or two solids. In immersion measurements one needs to consider the effect of stress on wave propagation from fluid to solid. This leads to change in propagation direction and energy redistribution. Also additional modes could be excited leading to stress-induced birefringence. These are all important phenomena which require rigorous quantitative description since the stress effect in general is very small. Another important problem is ultrasonic characterization of residual stresses in composite materials [1]. It involves wave propagation through an interface between layers with different properties and stress levels.