The electrochemistry of single-crystal electrodes, pioneered by Clavilier et al. [1], clearly shows that the electrochemical behavior of metal surfaces depends not only on their chemical composition but also, to a large extent, on their surface structure. The understanding of the importance of the latter factor has introduced into electrochemistry concepts such as surface reconstruction, epitaxy, etc., which were originally peculiar to ultrahigh-vacuum (UHV) surface science. At the same time, a notable effort has been devoted to the application of experimental techniques typical of UHV to electrochemical systems [2]: one of the most successful applications has certainly consisted in the use of the electrochemical STM which, only six years after its introduction as a UHV tool [3], proved able to yield atomic resolution on the surface of polarized electrodes, i.e. in-situ [4, 5]. Nowadays, even if further in-situ techniques have been developed, such as low-angle X-ray diffraction [6] and atomic force microscopy (AFM) [7], STM remains the most widespread technique, in view of its ability to determine the structure of surface features in the range of micrometers and on the atomic scale as well. STM is particularly useful in the investigation of surface defects, which are often responsible for electrochemical effects of importance in electrochemical applications. Some examples are given and briefly discussed below.

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