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 σ-hole and π-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. "Lewis acid–Lewis base interactions are practically ubiquitous in chemistry, both in the gas and condensed phases. Therefore this book is of fundamental interest. Dr Grabowski is a true expert in this topic." Manuel Yáñez Professor Emeritus, Autonomous University of Madrid, Spain "The study of non-covalent interactions is one of the fastest growing areas at the intersection of chemistry, physics, biology, and materials science. The rising number of publications on this topic with very different depths of analysis, which use the entire available spectrum of experimental and theoretical approaches and consider very different molecular systems, requires regular generalization of the results obtained. Sławomir J Grabowski presents such a review here." Ilya Shenderovich Head of NMR, University of Regensburg, Germany "This book is interesting for people working in the areas of organic chemistry, physical chemistry and physical organic chemistry. It will provide important information required for understanding of inter- and intramolecular interactions between Lewis acids and Lewis bases as well as the electron charge shifts between the interacting species. The chapters are well organized to cover both experimental and computational aspects as well as mechanics involved in those interactions." Tandabany Dinadayalane Associate Professor, Clark Atlanta University, USA "The book is a valuable summary of an important field, of interest to many researchers." Ivar Olovsson Professor Emeritus, Uppsala University, Sweden Dr Sławomir J Grabowski is Ikerbasque Research Professor at University of the Basque Country (UPV/EHU) and Donostia International Physics Center (DIPC), Spain. He has published more than 200 peer-reviewed papers, 15 book chapters and 3 books (one as author and two as editor), with a h-index of 58. His main interests are in non-covalent interactions, crystallography and quantum chemistry. He holds a MSc and PhD from the University of Warsaw, Poland and a DSc from the Technical University of łódź, Poland. He is a member of the editorial boards of Journal of Physical Organic Chemistry and Computational and Theoretical Chemistry.