A tutorial presentation is made in broad and general terms regarding the properties of the MF and HF portions of the radio spectrum as they pertain to and affect communication systems. The fine grain behavior in terms of amplitude and phase variations are presented in conjunction with the effects of fading periods, time and frequency spread, and atmospheric noise. A discussion of both theoretical and experimental bounds in error rate levels of digital systems as a function of the basic attributes of the ionospheric channel is undertaken in connection with the adaptive approach to communication system design. Two adaptive systems are described briefly in terms of their ability to cope with the time variant dispersive ionospheric channel.