The ionized material that constitutes plasma permeates almost all of the universe beyond the planets and their atmospheres and satellites. This book describes the linear theory of many different waves and instabilities that may propagate in a collisionless plasma. Electrostatic and electromagnetic fluctuations and a variety of instability sources are considered. Applications of the theory are discussed with respect to spacecraft observations in the solar wind, terrestrial magnetosheath, magnetosphere and magnetotail and at the bow shock and magnetopause. Tables at the end of most chapters summarize wave and instability nomenclature and properties, and problems for the reader to solve are interspersed throughout the text.