This text is devoted to the study of equilibrium and nonequilibrium states of infinite continuous systems in quantum statistical mechanics. The states of these systems are described by infinite sequences of statistical operators (reduced density matrices) or Green's functions which satisfy the infinite hierarchy of integro-differential equations. The investigation of these equations and constructing their solutions is the main subject of this work. Model systems in the theories of superconductivity and superfluidity and other exactly solvable models are studied in detail. The book should be of interest to mathematical and theoretical physicists and applied mathematicians interested in quantum statistical mechanics.