In this thesis, the chemical nature of strong and very strong hydrogen bonds, including resonance assisted hydrogen bond (RAHB), negative charge assisted hydrogen bond ((-)CAHB), and positive charge assisted hydrogen bond ((+)CAHB), is investigated. Different criteria such as energy, structural parameters, and Quantum Chemical Topology (QCT) analysis have been used to understand the nature of hydrogen bonds in these systems. In the first part, the RAHB systems have been considered. Based on the topological parameters of the Quantum Theory of Atoms in Molecules (QTAIM), the hydrogen bonding of these systems are partially covalent and partial electrostatic. Although QTAIM (as the basis of QCT) provides useful information, it cannot reveal all aspects of chemical bonds. Accordingly, to shed more light on the nature of these types of hydrogen bonds, we used Interacting Quantum Atoms (IQA) approach to partition the molecular energies into intra- and interatomic energy components. The IQA method was used to investigate the properties of atoms that involved directly in hydrogen bonding (i.e., ) as well as other interatomic interactions in the molecule. Since IQA can divide the interaction between two atoms in the molecule into two electrostatic and exchange interactions, it provides a quantitative picture of the covalent and electrostatic properties of each bond. In contrast to the QTAIM parameters, IQA results show that the hydrogen bonds in these systems are mainly electrostatic. The charge transfer component is the largest contributor to these electrostatic interactions. Both QTAIM and IQA calculations confirm the importance of resonance in the stability of RAHB systems. In the second part, hydrogen bonds in CAHB systems are considered. Although QTAIM parameters indicate that these hydrogen bonds are covalent, IQA calculations show that the electrostatic interactions are mainly responsible for the stability of these interactions.