The electrochemical nucleation and growth of metal clusters and low-dimensional metal structures can be achieved locally at predefined positions on a substrate surface by using the tip of a scanning tunneling microscope (STM) as an electrochemical nanoelectrode. The fermi level of this nanoelectrode can be easily controlled and very rapidly changed, which allows the generation of supersaturation or undersaturation conditions, with respect to the bulk metal equilibrium potential, in the electrolyte underneath the STM tip. Thus, by generating a supersaturation of metal ions around the STM tip, the metal/metal ion equilibrium potential is shifted to positive potentials within a certain substrate area underneath the STM tip. Due to the potentiostatic condition of the substrate, nucleation of metal atoms on the substrate surface is initiated. Nanostructures can be grown with a controlled aspect ratio and with lateral diameters even below 20 nm on metal and semiconducting substrates, as is demonstrated by the model systems Co on Au(111) and Pb on n-Si(111):H, respectively.