The subject of wave-particle interactions occurring in space plasmas has developed strongly, both observationally and theoretically, since the discovery of the Van Allen radiation belts of energetic charged particles trapped in the Earth's magnetosphere over forty years ago. These wave-particle interactions are recognized today as being a most important research topic in space plasma physics. This book provides a full and systematic description of the physical theory of whistler and Alfven cyclotron masers acting in planetary magnetospheres, and in the Sun's outer atmosphere. The book introduces research topics by examining significant problems in the subject. It gives sufficient detail on the topic that readers can go on to apply the methods presented to new problems, helping them with their own research. This book is a valuable reference for researchers and graduate students working in space science, solar-terrestrial physics, plasma physics, and planetary sciences.