This text is primarily concerned with the interaction of electrons with electromagnetic waves for the generation and amplification of radiation. It dicusses the various methods of electron-beam generation and propagation and the basic electron-wave interaction schemes, then it presents an idealized picture of the beam-wave interaction in a dielectric loaded waveguide. Gradually the complexity of the analysis is increased by presenting the effect of reflected waves and further by considering the beam as an ensemble of macro-particles. The book also dicusses the interaction in periodic and in quasi-periodic metallic structures and the fundamentals of the beam-wave interaction in a periodic field, concluding with a brief discussion of particle acceleration, including novel acceleration schemes.