In the field of molecular scale electronics the drive is towards the fabrication of self-assembled, organic, nanoscale architectures which will have an active role to play in novel electronic devices. As a formative step towards this goal the creation of an organic analogue to the p–n junction was proposed by Aviram and Ratner in the 1970s. In their proposal a monomolecular layer of a charge transfer species controls current flow between a pair of metal electrodes, allowing easy flow for only one polarity of the applied voltage. Such metal/molecular layer/metal structures have now been fabricated, utilising the self-ordering properties of Langmuir–Blodgett films to form the organic layer, with one dimension of the device being reduced to the molecular scale. The fabrication techniques involved in the generation of these M/LB/M junctions are now described along with the present understanding of conduction mechanisms through such nanoscale thickness junctions. These structures clearly show that the organic molecular layers can control current passage in electronic devices emulating some of the characteristics of an inorganic semiconducting p–n junction.