This classic graduate text, first published in 1961, studies the way that the interaction between particles in a dense system alters the behavior of the isolated, noninteracting particles. The three system properties of interest are: ground-state energy; elementary excitation spectra, including quasi-particles and collective modes; and various temperature-dependent phenomena for both equilibrium and nonequilibrium states. One of the first titles in the Frontiers in Physics series, The Many-Body Problem continues to be an important theoretical building block for physicists today.