When we were first approached by Dr. Lucky to write this book we were very enthusiastic about the prospect, since we had contemplated a similar project for quite some time. The difficulty lay in how best to digest the vast amount of data on optical propagation, reduce it to a book of manageable size, and simultaneously form the transition from the physics of propagation to the engineering of optical channels. This is the intent of Optical Channels. In accomplishing our goal it was necessary to condense the material on optical propagation and, in so doing, we have left a large amount to be handled via references. We have tried to make these decisions in a consistent manner so that the book will be uniform in its treatment of this topic. We identify four channels for consideration: the free-space channel, which: is characteristic of a tranquil atmosphere or a space-to-space link; the turbulent channel, which is characteristic of the atmospheric channel; the scatter channel in two forms, clouds and water; and the fiber optic channel. For each of these channels we have tried to reduce the applicable propagation theory to a level that can be used for engineering design. This has been done by example, but here again decisions had to be made on which examples to present. We have not tried to present any material on optical components and consequently other references on engineering would be necessary to supplement this book.