The course gives an introduction to the use of field-theoretical methods for a description of the equilibrium properties of many-body systems. The effects of both classical and quantum fluctuations are treated by methods involving many-body wave functions, diagrammatic expansions, and Hubbard-Stratonovich transformations. Particular topics discussed include Debye screening and plasma oscillations, Landau theory of phase transitions, superfluidity, superconductivity, and ferromagnetism.