Low-temperature STM does not only allow to image metallic surfaces in sub-nanometer resolution, but also to manipulate the adsorbates. Manipulation may be based on the electrical field in the tunnelling gap, on chemical forces between tip and sample or on electrons of the tunnelling current. The tunnelling parameters determine the desired manipulation mode. Our group uses tunnelling electrons in order to excite molecular vibrations that eventually lead to surface reactions. We have successfully formed and cleaved inter- as well as intra-molecular bonds. Furthermore, isomerisation reactions were induced. The latter are important in view of switches in molecular electronics. Published reactions were performed on water molecules. Formation of water clusters and cleavage of hydrogen bonds are due to excitation of the scissoring mode and also the OH stretch vibrations. To dissociate the water molecules, electrons are injected directly into the conduction band of a crystallite. Further data has been gathered for the isomerisation of chloronitrobenzene on Cu(111) and Au(111) and of the photochrome azobenzene derivate Disperse Orange 3 on Au(111).