This paper is Pt. I of a trilogy [Pt. II: T. G. Bell, “Field Data.” Paper K2, this meeting. Pt. III: H. W. Horton, “Model Studies.” Paper K3, this meeting]. Theoretical topics that are relevant to the most important effects observed in the field are discussed. These are grouped under intensity of reflection, distortion of reflected wave, and scattering, and include models predicting effects of absorbing bottoms, layered media, and irregular boundaries.