Recently, accurate ab initio quantum computational chemistry has evolved dramatically. In particular, the development of multireference-based approaches has opened up a whole new area and has also had a profound impact on the potential of theoretical chemistry.The multiconfigurational SCF (MCSCF)/CASSCF method is an attempt to generalize the Hartree–Fock (HF) model and to treat real chemical processes, where nondynamic correlation is important, while keeping the conceptual simplicity of the HF model as much as possible. Although MCSCF/CASSCF itself does not include dynamic correlations, it provides a good starting point for such studies. There are three approaches to handling dynamic correlations. Beginning with the MSSCF/CASSCF wave function, they are the variational (MRCI), perturbational (MRPT) and cluster expansion (MRCC) approaches.This important book presents the most recent and important developments in multireference-based approaches and their applications. Its main purpose is to highlight essential aspects of the frontiers of multireference theory and provide readers with the fundamental knowledge necessary for further development.