The first part of this book deals with a description of the development of the investigations on internal rotation. In the second part are presented a more detailed explanation of some of the theoretical aspects of the problem, especially that of normal vibrations, and a description of experimental methods. This is built up as systematically as possible so that this may be understood by the chemists whose special field is in other directions but who wish to increase their knowledge of the general field of structural chemistry. Therefore, it was necessary to include some sections well known to the specialists.