Multireference Møller–Plesset (MRMP) theory is an extension of the closed-shell single reference MP method and has been successfully applied to numerous chemical and spectroscopic problems. MRMP has established as an efficient technique for treating both dynamical and nondynamical correlation effects. Emphasis is put on the recent developments: MRMP in the extended basis function space (EBS) and relativistic MRMP with a new relativistic scheme by eliminating small-component (RESC) of the four-component Dirac equation. EBS-MRMP is a theory based on a concept of different bases for different correlation effects. The new RESC-scheme is a very promising alternative to the Dirac method.