This text by one of the originators of the cluster variation method of statistical mechanics is aimed at second- and third-year graduate students studying such topics as the theory of complex analysis, classical mechanics, classical electrodynamics, and quantum mechanics. The central theme is that, given the Hamiltonian for a system, it is possible to calculate the thermodynamics correlation function, either numerically or through the use of infinite series. The book is self-contained, with all required mathematics included either in the text or in Appendixes. The text includes many exercises designed for self-study.