Covering analytical research in aerial and underwater acoustics, this new scholarly work treats the interaction of acoustic waves with obstacles which may be rigid, soft, elastic, or characterized by an impedance boundary condition. The approach is founded on asymptotic high frequency methods which are based on the concept of rays. For despite the progress in numerical methods for diffraction problems, ray methods still remain the most useful method of approximation for analyzing wave motions. They provide not only considerable physical insight and understanding of diffraction mechanisms but they are also able to treat objects which are still too large in terms of wavelength to fall in the realm of numerical analysis.