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Galaxies are large systems of stars, some of which contain interstellar gas and dust. They also contain much invisible matter, which may be in the form of weakly interacting elementary particles. In this introductory textbook, the first chapter introduces the study of galaxies. This is followed by two chapters on observations of galaxies, 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 covered. The book concludes with a discussion of the origin of galaxies and their relation to more general questions in cosmology. The book includes mathematical presentation where this enables the discussion to be quantitative.