Substantial success has been achieved in statistical physics in the last few years as a result of the wide use of methods borrowed from quantum
field theory. The fruitfulness of these methods is connected with a new
formulation of perturbation theory and above all with the extensive use
of so-ealled Fcyiiinan diagrams. The main advantage of the diagram
technique lies in ease of visualisation based on the concepts of the single-
particle problem, it enables the structure of any approximation to be
established and the necessary expressions to be written down with the
aid of correspondence rules. The new methods are the most powerful and
effective ones in quantum statistics at the present time, and have made
it possible to solve a large number of problems which were insoluble in
the old formulation of the theory; moreover, many new general rela-
relationships have been obtained.