Information on the structure and surface reaction dynamics of an electrode/electrolyte interface is essential to reveal the reaction mechanisms and to control electrochemical reactions. Since the second order nonlinear optical processes such as second harmonic generation (SHG) and sum-frequency generation (SFG) are forbidden in the media of centro-symmetry, these techniques provide interface specific information and can be used to probe buried interfaces as far as they are accessible by light. We applied SHG and SFG to obtain information on electronic and molecular structure at the electrochemical interfaces and two examples are presented here. First, the electrochemical reaction HCHO at Pt surface was investigated by SHG spectroscopy. Next, the structure of interfacial water was characterized under potential control by SFG spectroscopy.