Inter- and intramolecular interactions that correspond to contacts between Lewis acid and Lewis base sites are considered in this monograph. Various types of interactions are described - halogen bond, pnictogen bond, hydrogen bond, etc. - and the mechanisms of these interactions as well as accompanying phenomena are presented. While we focus mainly on the ?-hole and ?-hole concepts that explain the majority of such interactions, recent ideas that the interactions may be treated as the preliminary stages of chemical reactions, as well as the notion that the formation of interactions is in agreement with the Valence Shell Electron Pair Repulsion model, are also discussed.Chapters are also dedicated to different experimental and theoretical approaches that are useful to analyze Lewis acid-base interactions. The crystal structures are the main source on molecular structures and interactions. Thus, we cover conventional experimental tools such as X-ray and neutron diffraction approaches, as well as newer methods for experimental electron density. An approach applied to analyze Hirshfeld surfaces is also described. On the computational front, the Quantum Theory of Atoms in Molecules (QTAIM) method, Non-Covalent Interactions (NCI) approach, Electron Localization Function (ELF) method, Natural Bond Orbital (NBO) approach, the Energy Decomposition Analysis (EDA), the Car-Parinello molecular dynamics (CPMD), and others are included.