Nonequilibrium Green's function techniques, initiated by Schwinger and Kadanoff and Baym allow ones to study the time evolution of a many-particle quantum sys- tem. Knowing the 1-particle Green's functions of a given system, one may evaluate 1-particle quantities like carrier density or current. The many-particle information about the system is cast into self-energies, parts of the equations of motion for the Green’s functions. Perturbation expansion of the Green's functions is the key to approximate the self-energies.
Green's functions provide a very powerful technique for evaluating properties of many-body system both in thermodynamic equilibrium and also in nonequilibrium situations.