After discussing experimental works on the measurement of the conductance of a metal–molecule–metal tunnel junction with many down to a few molecules and down to only one molecule in the junction, we propose a comprehensive way of understanding the electron transport phenomenon through such a junction. The dependence of the junction conductance on the length of the molecule(s) is explained starting from the quantum super-exchange electron transfer phenomenon up to the effective mass of the tunnelling electrons in the coherent limit. This super-exchange mechanism results from the electronic coupling between the two electrodes introduced by the molecule(s). The molecular wire guides this interaction better than the electronic coupling through vacuum between the two electrodes of the junction. Dephasing and thermal effects during the electron transfer events along the molecular wire are described using a density matrix formalism. The implication of our understanding of this through junction electronic transport is described starting from hybrid molecular electronics towards mono-molecular electronics.