This book provides material for courses in theoretical physics for undergraduate and graduate students specializing in condensed matter, including experimentalists who want a thorough theoretical background; the advanced part should be of interest to research workers too. A good first course in quantum mechanics is assumed. Here a variety of many-body phenomena in condensed matter are discussed, with special attention paid to the understanding of strong correlation effects. This requires a variety of theoretical tools (diagram expansions, groups, recursion methods and more). The text, which arose naturally from teaching, is eminently readable and the mathematical treatments are explained in enough detail to be followed easily. Proofs of all the relevant theorems are provided, but the main emphasis is always on the physical meaning or applicability of the results. Many examples are provided for illustration and also serve as worked problems.