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...