In the last twenty years, the impressive development of molecular electronics and its related technology made possible the establishment of molecular electronic sensors (MES). In particular, the advancements in molecular synthesis engineering and the evolution towards sub-photolithographic processes allowed to fabricate molecular sensors whose working principle is different from conventional molecular chemo-sensors. Indeed, in MES, the detection involves the electric current flow instead of other chemical-physical phenomena, thus making possible a direct integration with conventional electronic circuitry. Pollution, chemical compounds, humidity, temperature and light sensors are realizable by modulating the molecular channel conductivity, exploiting quantum phenomena such as intermolecular or photon interactions.