The ocean is transparent to sound where slight irregularities within the ocean cause sound fluctuations, and thus set limits on the many uses of sound in the ocean, similar to the limits imposed by the atmosphere on ground-based telescopes. This 1979 book attempts to connect the known structure of the ocean volume with experimental results in long-range sound transmission. Theories of wave propagation through irregular media, developed for optical and radio wave transmission are found to be inapplicable in many respects due to the complications of ocean structure, particularly the combination of anisotropy and 'sound channel'. The authors extend wave propagation theory to account for the ocean complications and introduces the path-integral approach to the solution of the strong-scattering regime that solves many long-standing problems. The book is written at the post-graduate level, but has been carefully organised to give experimenters a grasp of important results without undue mathematics.