The analysis of electronic relaxation processes, especially of radiationless transitions in molecular systems, has evolved rapidly over the last few decades and nowadays plays a central role in almost all investigations of molecular physics and spectroscopy. Advances in lasers have contributed significantly to this evolution, and the book provides a self-contained and unified presentation of this development, with applications to molecular and solid state physics. Primarily intended for graduate students in theoretical physics and chemistry, this is also indispensable reading for those working in molecular physics, physical chemistry and laser physics.