A difference between AC-DFT and multiconfigurational AC theory is that, in the former theory, the electron density is fixed to the exact density by a local one-body potential varying along the AC path, while in the latter, the condition of a constant density is imposed as an approximation. AC-based correlation energy approximations have been used with MCSCF, complete-active-space SCF (CASSCF), density-matrix-renormalization group (DMRG) or geminal theories. These multiconfigurational AC approximations rely on the adiabatic-connection fluctuation-dissipation formalism and the (extended) RPA. An appealing feature of the multiconfigurational AC methods is that only one-and twoelectron reduced density matrices are needed, as opposed to perturbation approaches such as complete-active space secondorder perturbation theory (CASPT2) or N-electron valence-state second-order perturbation theory (NEVPT2), which require threeand four-body RDMs.