  Recent experiments on mesoscopic normal-metal--superconductor
heterostructures resolve properties on length scales and at low temperatures
such that the temperature is below the Thouless energy $k_B T \le E_{Th}$. We
describe the properties of these systems within the framework of quasiclassical
many-body techniques. Diffusive and ballistic systems are covered, both in
equilibrium and nonequilibrium situations. Thereby we demonstrate the common
physical basis of various subtopics.
