In the years since the pioneering efforts of Sir Edward Appleton, M. A. F. Barnett, G. Breit, and M. A. Thve, many radio techniques have been employed to investigate the terrestrial ionosphere. The purposes of this book are to exam ine the basic physical interaction process of radio waves with the ionosphere, scrutinize each of the radio techniques currently in use, and describe the elements of each technique, as well as assess their capabilities and limitations. I have included some of the history of each technique, since we often tend to forget the efforts of the "pioneers." The interaction...