A companion volume to Conceptual Trends in Quantum Chemistry, this work contains eight contributions focusing on important conceptual trends in atomic and molecular theory. The polarization between ab initio and semi-empirical methods is thoroughly analyzed in two of the articles, which also provide bridges between such procedures. Hydrogen-transfer theory and electron delocalization are treated in two further papers. Explicitly time-dependent descriptions of intermolecular dynamics, which constitute a characteristic trend in current research, are represented by an article about the quantum...