Algebraic number theory is one of the most traditional and simultaneously particularly relevant foundational disciplines of mathematics today. This book addresses it in a detailed and comprehensive framework that includes both the fundamentals and their highlights. The presentation introduces the reader to the field in a concrete manner, guided by modern insights of a higher nature, and many parts are new. The foundational first part is enriched with some new aspects, such as a detailed theory of orders. Beyond the basics, the book includes a geometric re-establishment of the theory of algebraic number fields through the development of a Riemann-Roch theory from the Arakelov perspective, leading to a Grothendieck-Riemann-Roch theorem, as well as local and global class field theory and more.