Until recently, there had been few in situ methods available for the structural determination of an electrode surface, in solution, at the atomic level. Atomic level information had previously been acquired only via surface spectroscopic techniques in ultrahigh vacuum (UHV). However, since its invention by Binnig and Rohrer, scanning tunneling microscopy (STM) was immediately established as an invaluable and powerful surface analysis technique with atomic resolution in UHV. On the other hand, developments in STM operated at solid-liquid interfaces led to its valuation as the premier technique for atomic-level surface structural investigations of chemical processes taking place at solid-liquid interfaces. It has been demonstrated that in situ STM makes it possible to monitor, under reaction conditions, a wide variety of electrode processes such as the adsorption of inorganic and organic species, the reconstruction of electrode surfaces, the dissolution and deposition of metals and semiconductors. Several review articles on in situ STM and related techniques such as in situ atomic force microscopy (AFM) have been published [1-4]. The reviews by Gewirth and Niece [3] and the present author [4] are the most comprehensive in terms of results obtained on various substrates of metals and semiconductors. Due to publication limitations, the present abstract describes only headlines of our studies using in situ STM which have been listed in my previous review [4].

1. Scope of the Review
2. Introduction
3. Experimental Aspects
  A. Principle of In Situ STM in Electrolyte Solution
  B. Preparation of Well-defined Electrode Surfaces
4. Selected In Situ STM Studies
  A. Structure of Specifically Adsorbed Anions
  B. Underpotential Deposition
  C. Adsorption of Organic Molecules on Iodine-modified Electrodes
  D. Adsorption of Aromatic Molecules on Clean Bare Electrodes
  E. Site-selective Anodic Dissolution of Metals
  F. Electrochemical Dissolution Processes of Semiconductors

References
1. P. A. Christensen, Chem. Soc. Rev., 1992, 21, 197-208.
2. H. Siegenthaler, in Scanning Tunneling Microscopy II, (Eds.: R. Wiesendanger, H.-J. Guntherodt), Springer-Verlag, Berlin, 1992, pp. 7-49.
3. A. A. Gewirth, B. K. Niece, Chem. Rev. 1997, 97, 1129-1162.
4. Itaya, Prog. Surf. Sci. 1998, 58, 121-247.