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Relativistic effects play a pivotal role in describing the electronic structure ofmolecules and materials. Although there has been rapid advancement in implementingelectronic structure theories with spin–orbit and scalar relativistic effects, accurateand efficient implementation of such effects with high-level correlation methods arestill under-developed. In this thesis, we will present the development of relativisticmultireference perturbation theories that are necessary to describe the electronicstructure of strongly correlated molecules. First, I will present the implementationof spin–orbit coupling effects within the framework of quasi-degenerate N-electronvalence perturbation theory (QD-NEVPT2) where I demonstrate its capabilities insimulating electronic structure of atoms and molecules with strong spin–orbit couplingeffects. Next, I will present QD-NEVPT2 with consistent second-order treatment ofspin–orbit coupling effects and dynamical correlations using a more accurate spin–orbitcoupling Hamiltonian where I will demonstrate the necessity of this theory to describeelectronic structure of elements belonging to 5th and 6th rows of the periodic table.Finally, we I implement relativistic effects in multireference algebraic diagrammaticconstruction theory (MR-ADC) to describe spin–orbit coupling effects in chargedexcitation processes (electron ionization and attachment).