In the first part of the talk the statistical operator and the Master equation are introduced in order to describe electronic transport through a mesoscopic sample connected to external leads. The contacts between the sample and the leads are modeled with time dependent functions which simulate local potential barriers. The sample contains only a few electrons and the Coulomb interaction is fully included within the exact-diagonalization scheme. In the second part of the talk recent examples of transport calculations in quantum dots, nanowires, and double systems are presented. Effects of an external electromagnetic field on electronic transport are also shown. The seminar will be theoretical, but the presentation will be introductory and guided by physical intuition.