What is Supersymmetry? Is it something real? If not, can it be useful in any way? This book, structured as a textbook for a one semester graduate course on supersymmetry, provides an introduction to this fascinating subject and seeks to answer these questions. Theoretically inclined in its contents, the book can be divided into three parts. The first part introduces the supersymmetry algebra and its representations, and provides a detailed description of the superfield formalism. The second part focuses on the construction of supersymmetric field theories; it includes an overview on non-renormalization theorems, the analysis of several examples of tree-level supersymmetry breaking and a discussion of the basic structure of supersymmetric models for describing physics beyond the Standard Model. The third part discusses the quantum behavior of supersymmetric (gauge) theories, in which holomorphy and dualities play a prominent role. The reader will become familiar with topics like Seiberg duality, dynamical supersymmetry breaking (both in stable and metastable vacua), Seiberg-Witten theory, Argyres-Douglas fixed points, S-duality and more. Several exercises at the end of each chapter will allow readers to test their understanding, discuss some extensions, or prove statements from the main text.