This book is intended as a pedagogical introduction to the rapidly developing field of the AdS/CFT correspondence. This subject has grown to the point where graduate students, as well as researchers, from fields outside string theory or even particle theory, in particular nuclear physics and condensed matter physics, want to learn about it. With this in mind, the book endeavours to introduce AdS/CFT without assuming anything beyond an introductory course in quantum field theory. Some familiarity with the principles of general relativity, supersymmetry or string theory would help the reader follow more easily, but is not necessary, as I introduce all the necessary concepts. I do not overload the book with unnecessary details about these fields, only what I need to give a simple, yet completely rigorous, account of all the basic methods, tools, and applications of AdS/CFT. For more details on these subjects, one can consult a number of good textbooks available for each, which I suggest at the end of the corresponding chapters. When explaining AdS/CFT, I try to give a simple introduction to each method, tool, or application, without aiming for an in-depth or exhaustive treatment. The goal is to introduce most of the AdS/CFT methods, but for an in-depth treatment one should refer to research articles instead. Part I of the book deals with the necessary background material, so someone familiar with this can skip it. Part II describes the basics of AdS/CFT in the context of its best understood example, N = 4 SYM vs. string theory in AdS5 × S5. Part III deals with more specialized applications and other dualities, generalizing to the gauge–gravity dualities.