The multireference, state-specific, second-order, Brillouin-Wigner perturbation theory is presented as a robust approach to the electron correlation problem for systems demanding the use of a multireference function. A posteriori modifications are made which, in the case of a single reference function, recover the well known formula of second-order many-body perturbation theory (MBPT2), i.e. Moller-Plesset (MP2) theory, and in the multireference case can be shown to be equivalent to state-specific multireference Rayleigh-Schrodinger-like perturbation theory. It is shown that multireference many-body Brillouin-Wigner perturbation theory when restricted to second-order is a true many-body theory, that is, it is a theory, which has the property of extensivity. (c) 2007 Wiley Periodicals, Inc.