This textbook introduces the molecular physics and quantum chemistry needed to understand the physical properties of molecules and their chemical bonds. It follows the authors' earlier textbook "The Physics of Atoms and Quanta" and presents both experimental and theoretical fundamentals for students in physics and physical and theoretical chemistry. The new edition treats new developments in areas such as high-resolution two-photon spectroscopy, ultrashort pulse spectroscopy, photoelectron spectroscopy. Particular emphasis is laid on the optical investigation of single molecules in condensed phase. Finally, the developing field of molecular electronics is presented, including electroluminescence and light-emitting diodes.