Book excerpt: The interaction of electromagnetic waves with matter has always been a fascinating subject of study. As matter in the universe is mostly in the plasma state, the study of electromagnetic waves in plasmas is of importance to astrophysics, space physics and ionospheric physics. The physics of electromagnetic wave interacting with electron beams and plasmas also serves as a basis for coherent radiation generation such as free electron laser and gyrotron and advanced accelerators. This monograph aims at reviewing the physical processes of linear and nonlinear collective interactions of electromagnetic waves with electron beams and unmagnetized plasmas.