Intermolecular complexes play an important role in physical chemistry, molecular physics and, especially, in the biodisciplines. In the early 1980s, rapid and significant progress was made in both experimental and theoretical investigations of molecular interactions - a fact which encouraged the authors to prepare this monograph. The coverage of the subject is particularly comprehensive, ranging from theory to experiment, from interactions of small systems to very large systems, as well as interactions of large numbers of systems, and from chemical to biological applications. The book is written for students and scientists working in various areas of chemistry, biology and physics. They will appreciate the fact that quantum and other theories of molecular interactions, experimental investigations of molecular interactions, and applications in physical chemistry and biodisciplines are consistently covered within a single book. Different computational techniques are analysed and concrete recommendations of computational strategy made for the treatment of van der Waals systems of various size.