Ions are atoms or molecules stripped of their electrons, so they can be accelerated by electric fields. They can be made to hit each other with low energy, intermediate energy, high energy, or very high energy; each energy range seeks to investigate different aspects of hadronic physics. Intermediate-energy heavy ion collisions explore the nuclei far from stability valley, the incompressibility of nuclear matter, the liquid-gas phase transition in nuclear environment, the symmetry energy far from the normal density, and other phenomena. This has been an active field of research for last four decades.This is a book for entrants in the field. It is suitable as a companion book in a graduate course. For practitioners in the field it will be useful as a reference.