The active dissolution of a bare metal surface is of central importance for technological processes, such as aqueous corrosion, etching, and electrorefinement. Even on clean, single crystalline electrodes this electrochemical reaction proceeds at a highly non-uniform rate and is strongly localized at a small number of atomic-scale defects ("active sites") on the metal surface, which are commonly identified with kinks in atomic steps on the crystal surface. With the help of in situ scanning tunneling microscopy (STM) a detailed picture of the underlying microscopic mechanisms and dynamics is now emerging. Here, results on the dynamics of dissolution and growth at the solid-liquid interface by novel, STM-based methods with high temporal and spatial resolution are presented. Using high-speed Video-STM with image acquisition rates of up to 25 images per second the underlying atomic-scale processes, such as the nucleation and propagation of kinks along the steps of the crystal surface, can be directly studied, as illustrated for Cu(100) in HCl solution. In particular, it will be shown how the ordered Cl adlayer on this electrode surface determines the structure of active kinks as well as local reaction rates at these sites. Quantitative measurements of the kink propagation rates, i.e., the local dissolution/redeposition rates, reveal high local rates and pronounced local dissolution/redeposition fluctuations at the individual kinks. Even at the onset of Cu dissolution the average kink propagation rates and the average reaction rates at kink sites are in the range of 10@super 3@ and 10@super 5@ atoms s@super -1@, respectively. In addition, these studies give insights into the mechanisms of kink formation during metal dissolution/deposition, which can explain the characteristic surface morphology in this system. Together, these data give a coherent picture of the atomic-scale processes involved in the electrochemical dissolution and growth of Cu(100) in HCl solution.