This book is an outgrowth of material covered in a graduate course on ionospheric radio propagation offered since 1959 by the Department of Electrical Engineering, University of Illinois at Urbana-Champaign. Some material was also used in a special problems course on propagation in inhomogeneous media. The book is intended primarily as a graduate text for students in radio science, plasma physics, wave propagation, ionospheric physics, and atmospheric science. As such, theories and ideas are stressed more than engineering practice. Problems have been included to illustrate and amplify these theories and ideas. Some of these problems have been taken from the published literature and are by no means trivial; some others are fairly simple. We anticipate that the book may be of interest as well to researchers in the field. For their benefit, a list of references are attached at the end of each chapter; this list is far from being complete. We wish to apologize to those authors upon whose work we may have drawn but neglected to acknowledge because of oversight.