[en] We consider mesoscopic structures containing normal metal-superconductor interfaces. The interplay between superconducting coherence effects (Andreev reflection) and mesoscopic effects known to occur in diffusive normal metal, give rise to various interesting phenomena in these structures. In particular, tile effect of a barrier at all NS interface on the conductance of the system and on tile local density of states (DOS) across it is different than what would be naively expected. We use scattering theory to show that even if the barrier has transmission probability Γ >> 1, it can be effectively transparent. Consider a physical property which is determined by a certain set of electron trajectories in the normal metal. If most of these trajectories hit the interface more than f-l times before electron-hole coherence is lost then the barrier is ineffective. We show that from this criterion alone one can reproduce the different conditions for the barrier to be effective in both problems of the conductance of the system and the local DOS with and without magnetic field. We also apply this criterion to ballistic systems where the geometry allows multiple reflections from the NS interface. Another interesting phenomena is the nonmonotonic differential conductance of an NS junction as a function of energy (reentrance effect). We use a scattering formalism to show that in the case of finite transmission through the normal metal part, at the Fermi energy, the different paths that contribute to the Andreev reflection process interfere destructively. Therefore, at finite energies the destruction of the electron-hole coherence results in an enhancement of the Andreev reflection probability, and therefore an enhancement of the differential conductance of the NS system, which is the main feature of the reentrance effect