This book gives an overview of relativistic heavy ion physics with particular emphasis on those theoretical approaches which seek an understanding and explanation of the measurements. These approaches try to build a bridge between more basic theories, such as lattice QCD or nucleon-nucleon interactions, and complicated experimental observables involving a large number of particles. Thus, mainly theoretical approaches are discussed here which are strongly and directly related to experiments, and in turn they are phenomenological to some extent. These models use the available information from more complete reaction model describing the whole collision and the observables.It is suitable as a text for advanced undergraduate and graduate students - both experimentalists and theorists - for studies in the field of relativistic heavy ion physics. It may also serve as a handbook where basic concepts of reaction models can be found and the most important references for further reading are provided.