  Thermodynamic and transport properties of mesoscopic conductors are strongly
influenced by the proximity of a superconductor: An interplay between the large
scale quantum coherent wave functions in the normal mesoscopic and the
superconducting region, respectively, leads to unusual mechanisms of quantum
interference. These manifest themselves in both the mean and the mesoscopic
fluctuation behaviour of superconductor/normal-metal (SN) hybrid systems being
strikingly different from those of conventional mesoscopic systems. After
reviewing some established theories of SN-quantum interference phenomena, we
introduce a new approach to the analysis of SN-mesoscopic physics. Essentially,
our formalism represents a unification of the quasi-classical formalism for
describing {\it mean} properties of SN-systems on the one hand, with more
recent field theories of mesoscopic {\it fluctuations} on the other hand. Thus,
by its very construction, the new approach is capable of exploring both
averaged and fluctuation properties of SN-systems on the same microscopic
footing. As an example, the method is applied to the study of various
characteristics of the single particle spectrum of SNS-structures.
