The research and development of recent years on molecular electronic systems, in which the properties of devices are dictated by a molecule, has brought to the creation of a variety of structures. Thanks to the intrinsic characteristics of molecules, devices with a small footprint, and therefore portable and ready to be used, may be realized. Furthermore, the great tunability simply provided by a different choice of the employed molecule allows to build systems according to needs. As a consequence, an interesting area of study in molecular electronics is represented by gas sensors, where the possibility of having devices showing high selectivity with respect to desired targets is the main goal.