"I have endeavoured to give a presentation which is readable by the beginner in the field and also will be useful to those who do or want to do research work in this field. In order to assist the former, I have frequently made use of small type for those sections that are not necessary for an understanding of the fundamentals. For the benefit of those working in the field, numerous references to original papers have been included. "A satisfactory presentation of molecular spectra and molecular structure is nowadays not possible without treating thoroughly, apart from the empirical results, the theoretical background also. Therefore, I have included as much of the theory of molecular spectra as is possible without going into the more difficult mathematical details. A large number of diagrams, graphical representations of eigenfunctions and potential curves, as well as energy level diagrams, serve to illustrate and to explain the theory. On the other hand, I have added numerous carefully selected spectrograms of bands and band systems (some of which have been taken specially for this purpose) in order to give an accurate idea of the experimental material that forms the basis of the developments.