This volume contains a series of lectures delivered at the 2nd course of the International School of Astrophysics at the "Ettore Majorana" Centre for Scientific Culture in Erice (Sicily) from June 22 to July 9, 1974. The course was jointly planned by L. Woltjer and myself and was fully supported by a grant from the NATO Advanced Study Institute Programme. It was organized with the aim of providing students and young researchers with an up to date account of the structure and evolution of galaxies and was attended by 94 participants from 20 countries. The study of galaxies is one of the most important areas of contemporary astrophysics both for its intrinsic interest and because it is a prerequisite to a quantitative understanding of the structure of the universe. Recently, a qualitatively new under­ standing has become available through both observational andtheore­ tical progress. On the observational side, new techniques applied at the large optical telescopes and, in particular, the impressive results obtained wi. th the high resolution radio telescopes have made it possible to obtain a detailed mapping of the structure of galaxies other than our own. At the same time, new theoretical insights and the availability of powerful computers to construct models holds out the hope that a full understanding of the structur. es of galaxies may be within our reach.