This paper takes a broad look at the problems and techniques involved in the theoretical prediction of propagation loss in the ocean. Most propagation loss models in current use are based on ray theory, because of the relative simplicity with which ray solutions can be obtained and computations performed, even for range-dependent environments. However, ray theory is only an approximate solution of the wave equation and can lead, particularly at low frequencies, to erroneous predictions in sound channels and in the vicinity of caustics and shadow zones. The classical approach to exact solutions has been in terms of normal modes, based on separation of variables in a horizontally stratified environment. Developments in recent years have significantly advanced the state-of-the-art. With the application of wave corrections, ray models can now be extended to compute the field in the vicinity of caustics and shadow zones. In the wave approach, improved models have been developed using normal modes and other techniques, approximate solutions have been applied to range-dependent environments, and an efficient technique has been developed for the direct numerical integration of a modified form of the wave equation.