This book is a quantitative introduction to what is known or theorized about the structure and evolution of galactic systems. It begins with a general introduction to galaxies and a summary of our empirical observations of galaxies in the universe, including our own. There are then three chapters on galactic structure: the manner in which motions of stars determine galactic shape, the determination of galactic masses, and the structure of discs in spirals. Galactic evolution, especially changes in chemical composition over time, is then covered. The book concludes with a discussion of the origin of galaxies and their relation to more general questions in cosmology. The presentation is sufficiently mathematical so that quantitative results can be discussed in detail. Throughout, the author stresses what are currently accepted results and what theories may need revision in what continues to be a rapidly developing subject.