In this lecture, we explore transport properties of quantum systems whose dimensions are smaller than the characteristic phase coherence length. Below this length scale, the quantum nature of the electronic wavefunction shows up. In such systems, the complex interplay between disorder, Coulomb interaction and dimensionality results in original behaviours. By using Landauer formalism, we show how quantum statistics and electronic interferences affect the transport and break the classical laws for electricity. We see also how non-equilibrium current fluctuations are related to the fundamental properties of these systems. Using Green’s function formalism, we study how the interplay between electronic interferences and disorder leads to universal conductance fluctuations, weak localization and even to persistent currents at equilibrium. Finally, we investigate the interplay between such a coherent system and a superconductor, which yields new fundamental transport mechanisms.