The more prominent spatial and temporal irregularities of the ionosphere, interpreted from the abnormalities manifested by ionospherically propagated HF radio signals, are reviewed in Chapter I. Some of the applications of geometrical optics and diffraction theory of this problem are then presented. This is followed by a survey of various transionospheric , as well as vertical and oblique incidence, reflection investigations of the fluctuation of intensity (fading) of radio waves and the information derived there from concerning both the ionospheric irregularities and the structure of the radio wavefield at the ground. ...