The introduction of quantum field theory methods has led to a kind of ?revolution? in condensed matter theory. This resulted in the increased importance of Feynman diagrams or diagram technique. It has now become imperative for professionals in condensed matter theory to have a thorough knowledge of this method. There are many good books that cover the general aspects of diagrammatic methods. At the same time, there has been a rising need for books that describe calculations and methodical ?know how? of specific problems for beginners in graduate and postgraduate courses. This unique collection of lectures addresses this need. The aim of these lectures is to demonstrate the application of the diagram technique to different problems of condensed matter theory. Some of these problems are not ?finally? solved. But the development of results from any section of this book may serve as a starting point for a serious theoretical study.