The fourth book in the "Astronomy and Astrophysics" series contains an overview of modern ideas about giant star systems - galaxies. The history of the discovery of galaxies, their main types and classification systems are described. The basics of the dynamics of stellar systems are given. The galactic neighborhoods closest to us and work on the global study of the Galaxy are described in detail. Data are presented on various types of galaxy populations - stars, interstellar medium and dark matter. The features of active galaxies and quasars are described, as well as the evolution of views on the origin of galaxies. The book is aimed at junior students of university natural science faculties and specialists in related fields of science. The book is of particular interest to astronomy lovers.