In the interest of speed and economy the notation of the original text has been retained so that the cross product of two vectors A and B is denoted by AB], the dot product by (AB), the Laplacian operator by b., the curl by rot, etc. It might also be worth pointing Out that the temperature is frequently expressed in energy units in the Soviet literature so that the Boltzmann con stant will be missing in various familiar expressions. In matters of termin ology. whenever possible several forms are used when a term is first introduced. e.g . magnetoacoustic and magnetosonic waves, "probkotron" and mirror machine, etc. It is hoped in this way to help the reader to relate the terms used here with those in existing translations and with the conventional nomen clature. In general the system of literature citation used in the bibliographies follows that of the American Institute of PhYSics "Soviet PhysiCS" series; when a translated version of a given citation is available only the English translation is cited, unless reference is made to a specific portion of the Russian version. Except for the correction of some obvious misprints the text is that of the original. We wish to express our gratitude to Academician Leontovich for kindly providing the latest corrections and additions to the Russian text, and espe cially for some new material, which appears for the first time in the Ameri can edition."