A newly designed electrochemical scanning tunneling microscope (EC-STM) enables the combination of STM and electrochemical measurements (e.g. cyclic voltammetry) within a realistic volume of liquid (electrolyte). Furthermore three different detection modes with the STM, namely potentiostatic, potentiodynamic and spectroscopic, open the possibility to monitor surface structures, structural transition as well as absolute adsorption sites with subatomic resolution. In this lecture a description of the fully homemade electrochemical STM will be given, and its high in-situ performance will be demonstrated by some selected examples including the atomic structure of anionic adsorbates, the morphological changes of the respective electrode surface, the deposition of metal layers and the growth of molecular films.