The fundamental atomic-scale processes during active metal dissolution-the local removal/addition of atoms at atomic kinks in the steps on the crystal surface-were directly studied by novel instrumental methods based on in-situ scanning tunneling microscopy (STM), using the dissolution of Cu(100) in 0.01 M HCl solution as an example. Direct observation of these rapid dynamic processes with a novel high-speed electrochemical STM (Video-STM), capable of acquiring up to 25 atomic resolution images per second, reveals that metal dissolution proceeds at a single type of structurally well-defined kinks. The reactivity of these kinks is tentatively explained by the coordination of Cu surface atoms and Cl adsorbates at these sites. The kinks nucleate predominantly at outer terraces corners and can condense into larger facets, resulting in an apparent 'collective' local dissolution or growth of terraces. Quantitative data on the kink dynamics were obtained by an alternative approach (TOW-STM) and show pronounced local dissolution/redeposition fluctuations at the individual kinks even at the onset of Cu dissolution with average kink propagation and reaction rates at kinks in the range 10(3) and 10(5) atoms s(-1) respectively.