This book provide a unique introduction to the theory of scanning tunneling microscopy and related scanning probe methods. It is written by experts who have developed the theoretical foundations for understanding the contrast mechanisms involved in the various local probe methods. An overview of the different theoretical approaches is presented which will be helpful for every theorist starting in this field. On the other hand, the implications fo the theoretical results are discussed in detail. Therefore, this book will serve as a most useful guide for experimentalists as wll, facilitating the interpretation of their data.