This study considers the instabilities that result when an electron beam is injected into a plasma. A number of different models of the system are considered, and all instabilities are classified according to whether they are convective instabilities (amplifying waves) or nonconvective (absolute) instabilities. The study also analyzes the instabilities in unbounded beam-plasma systems and in systems of finite extent transverse to the electron stream and gives a detailed consideration of the possibility of a strong interaction with the ions in a hot-electron plasma. In addition, the author...