Book excerpt: This report contains a review of the work done on frequency fluctuations of ionospheric radio echoes on high frequencies. The historical aspects are briefly discussed to show the development of research on solar flare effects and the motion of ionospheric irregularities. The recording program over the past 7 years and the equipment modifications involved are discussed. This is followed by a brief review of the elementary theory underlying frequency fluctuations. In most of the report the mathematical equations are listed in tables rather than distributed throughout the text. Solar flare effects (called sudden frequency deviations) are used to determine the relaxation time of the ionosphere.