This book compiles the notes from a course in many-body physics, presented for nearly 20 years to graduate students at the University of Salento held by the author. It is intended for students already familiar with basic non-relativistic quantum mechanics. Building on this foundation, the book explores the challenges that arise when traditional quantum mechanics approaches are applied to systems composed of many interacting particles, such as nuclei, electron gas, multi-electron atoms, complex molecules, and quantum liquids. The first part of the book introduces the quantum many-body problem and provides essential information. The second part describes the Monte Carlo approach which solves the problem without approximations and its technical limitations are emphasized. In the third part, the approaches inspired by quantum field theory are presented. The theoretical entities used here are creation and destruction operators, Green's functions, and propagators. The theories are focused on the modifications of the interaction between the particles when they interact in a medium. In the fourth part, various theories inspired by statistical mechanics are discussed, with an emphasis on the correlations between particles in the system. The fifth part covers phenomenological theories, where the interaction between particles is defined within the framework of the theory itself to describe certain properties of the many-body system. The aim of the book is to provide graduate students with a comprehensive overview of the fundamental ideas related to the theories used to address the quantum many-body problem. Since the text is intended for students rather than experts in the field, detailed calculations are presented, with a focus on the basic concepts underlying the various theories rather than on recent numerical advancements, which are continually evolving.