This paper is concerned chiefly with ray and normal mode models. The deterioration in the performance of ray models at low frequencies is illustrated with examples of shadow zones, sound channels, and caustics. As the frequency decreases, the acoustic properties of the bottom become increasingly important, and in constructing wave models for very low‐frequency applications it is necessary to take the properties of the bottom realistically into account. A normal mode model is used to show how the maximum depth to which the bottom properties must be known increases with decreasing frequency, and to illustrate the effects of shear waves. The paper concludes with a discussion of the validity of neglecting the coupling of compression and shear waves in homogeneous sediments.