The structure of Au(100) surfaces in contact with aqueous solutions has been studied by in-situ scanning tunnelling microscopy. It is shown that the surface topography of flame annealed Au(100) sensitively depends on the sample preparation as well as on the electrochemical pretreatment. Flame annealed Au(100) surfaces are reconstructed and it is demonstrated under which conditions this initial (thermally induced) reconstruction is preserved during and after contact with the electrolyte. Once the initial reconstruction of a gold surface has disappeared, e.g. due to specific adsorption of anions, a potential induced reconstruction may take place, usually at potentials clearly negative with respect to the potential of zero charge. As a consequence, the surface structure of an Au(100) electrode is potential dependent and structural changes may occur within the potential region chosen for an electrochemical experiment. The kinetics and a mechanistic model of the structural transition are briefly discussed.