The book covers several methodologies used in various problems of classical and quantum statistical mechanics, e.g. mean field theories, scaling theory, exact methods especially Bethe Ansatz, numerical methods and renormalization group. These techniques are then used to understand more complex phenomena like multicritical cases, polymers, etc. Many models used in statistical mechanics are introduced both to show the applicability of the methods and as introduction to the models. The extension to nonequilibrium situation is discussed. An introduction to various quantum problems, like the Hubbard model, and anyons are also given. The basic steps and ideas of many body theory as applied in condensed matter physics are also spelled out.