A microscopic approach to the theory of small, current-biased tunnel junctions is developed. This approach yields a natural account of the “secondary” quantization of both the single-electron (quasiparticle) and Cooper-pair (Josephson) current components. The theory shows that the current of the single electrons is blocked by their Coulomb interaction at low temperatures within a considerable range of the junction voltage. As a result of the blockade, coherent oscillations of the voltage can arise even in the absence of Josephson coupling, e.g., for single-electron tunneling (SET) between normal metal electrodes. The most significant features of these “SET” oscillations and their coexistence with Bloch oscillations in Josephson junctions are studied in detail. Prospects of experimental verification of the predicted effects and of their possible applications are discussed.