Ultracold atomic gases is a rapidly developing area of physics that attracts many young researchers around the world. Written by world renowned experts in the field, this book gives a comprehensive overview of exciting developments in Bose-Einstein condensation and superfluidity from a theoretical perspective. The authors also make sense of key experiments from the past twenty years with a special focus on the physics of ultracold atomic gases. These systems are characterized by a rich variety of features which make them similar to other important systems of condensed matter physics (like superconductors and superfluids). At the same time they exhibit very peculiar properties which are the result of their gaseous nature, the possibility of trapping in a variety of low dimensional and periodical configurations, and of manipulating the two-body interaction. The book presents a systematic theoretical description based on the most successful many-body approaches applied both to bosons and fermions, at equilibrium and out of equilibrium, at zero as well as at finite temperature. Both theorists and experimentalists will benefit from the book, which is mainly addressed to beginners in the field (master students, PhD students, young postdocs), but also to more experienced researchers who can find in the book novel inspirations and motivations as well as new insightful connections. Building on the authors' first book, Bose-Einstein Condensation (Oxford University Press, 2003), this text offers a more systematic description of Fermi gases, quantum mixtures, low dimensional systems and dipolar gases. It also gives further emphasis on the peculiar phenomenon of superfluidity and its key role in many observable properties of these ultracold quantum gases.