The subject of radiative transfer has matured to the point of being a well-developed tool, which has been adapted over the years to a host of disciplines, ranging from atmospheric and ocean optics to stellar atmospheres. It has also become a part of many engineering curricula, since its industrial applications (particularly for the infrared) are wide ranging. As a result of this broadness, developments of radiative transfer theory in many separate fields have grown up in isolation. In comparing the literature in these various disciplines, one finds a bewildering multiplicity of approaches, which often obscures the fact that the same fundamental core is present, namely the radiative transfer equation. The same can be said for the two fields of atmospheric radiation and ocean optics. These have evolved along largely separate paths, with their own sets of jargon and nomenclature. However, in view of the fact that there is a growing need for interdisciplinary research involving the coupled atmosphere–ocean system, we feel that the time has come to write a textbook that acknowledges the following basic fact: The radiation that enters, or is emitted by, the ocean encounters the same basic processes of scattering and absorption as those involved in atmospheric radiation. There are no inherently different optical properties between atmospheric and aqueous media. Because the two media share a common interface that readily passes radiative energy, there is even more need for a unified approach.