The electronic tunnel factor in homogeneous long-range electron transfer (ET) systems has been characterized experimentally and theoretically in great detail. Outstanding questions, however, remain, for example interference in multiple ET routes and environmentally induced barrier fluctuations. Well characterized electrochemical long-range ET systems have also become available where the tunnel factor can be accurately deconvoluted from the total current. Intriguing new effects here are associated with the electrode charge or overpotential effects on the tunnel factor and with the feasibility of observation of critical current behaviour near phase transitions in thin films across which electron tunnelling occurs. In situ scanning tunnel microscopy (STM) offers a conceptual and technical new frame for mapping of molecular ET routes through large adsorbate molecules with low-lying local fluctuating levels. Such a configuration would be representative of large transition metal complexes or redox metalloproteins. High-resolution images and new theoretical approaches to STM mapping of large adsorbates are, moreover reported. The independent in situ electrode potential control provides new, non-monotonous features in the current-voltage relations which differ from electrochemical ET behaviour at a single interface.