Since the 1980s, single-crystal Pt electrodes with well-defined surface structures have been deemed stable under mild electrochemical conditions (e.g., in the potential region of electric double layers, underpotential deposition of hydrogen, or mild hydrogen evolution/OH adsorption) and have served as model electrodes for unraveling the structure-performance relation in electrocatalysis. With the advancement of in situ electrochemical microscopy/spectroscopy techniques, subtle surface restructuring under mild electrochemical conditions has been achieved in the last decade. Surface restructuring can considerably modify electrocatalytic properties by generating/destroying highly active sites, thereby interfering with the deduction of the structure-performance relation. In this review, we summarize recent progress in the restructuring of well-defined Pt(-based) electrode surfaces under mild electrochemical conditions. The importance of the meticulous structural characterization of Pt electrodes before, during, and after electrochemical measurements is demonstrated using CO adsorption/oxidation, hydrogen adsorption/evolution, and oxygen reduction as examples. The implications of present findings for correctly identifying the reaction mechanisms and kinetics of other electrocatalytic systems are also briefly discussed.