This treatment of molecular and atomic physics is primarily meant as a textbook. It is intended for both chemists and physicists. -It can be read without much knowledge of quantum mechanics or mathematics, since all such details are explained-. It has developed through a series of lectures at the Royal Institute of Technology. The content is to about 50 % theoretical and to 50 % experimental. The reason why the authors, who are experimentalists, went into theory is the following. When we during the beginning of the 1970's measured photo- electron spectra of organic molecules, it appeared to...