Lectures on the Many-Body Problem is a compilation of papers delivered at the Fifth International School of Physics, held at Ravello, Italy in April 1963. The book is devoted to the techniques of many-body theory, which are used in finding solutions to difficult problems encountered in solid-state physics. The text discusses such topics as the discontinuities in the drift velocity of ions in liquid helium; density fluctuation excitations in many-particle systems; tunneling from a many-particle point of view; the mathematics of second quantization for systems of fermions; and correlation functions and macroscopic equations. Theoretical physicists will find the monograph invaluable.