Weak interactions involving closed-shell molecules, such as hydrogen bonding, van der Waals forces, and π–π stacking, are pivotal to understanding a wide array of phenomena in chemistry, physics, and biology. These interactions dictate the behavior and properties of molecular assemblies, from protein folding and DNA stability to material properties and reaction mechanisms. Computational studies have become essential tools for investigating these interactions, providing detailed insights into their nature, strength, and effects. Modern computational methods enable researchers to model complex systems with high precision, decompose intermolecular energies into meaningful components, and design innovative materials and drugs. By bridging theory and experiment, computational approaches advance our understanding of intermolecular forces, driving progress across scientific disciplines. This chapter introduces a relatively new scheme for decomposing intermolecular interaction energy into distinct, physically meaningful terms – such as electrostatic, exchange, spin repulsion, and polarization – using the fragmentation of the complex’s electron density. The primary advantages of this method are its simplicity and low computational cost, making it highly practical compared to other techniques. Following an introductory section that reviews widely used methods in the theoretical community, three subsequent sections delve into the theoretical framework of this electron density-based energy decomposition analysis and evaluate its performance through previously reported results. These sections focus on the original scheme’s application to SCF and post-SCF levels; its extension to decompose polarization energy into induction and dispersion terms, and its use with large complexes in the context of DFT; and, finally, its generalization for application with QM/MM hybrid methods.