Our group focuses on employing the state-of-art second order nonlinear spectroscopic tools to study the microscopic structures, molecular excited states and chemical reaction dynamics at various surfaces or interfaces. The second order nonlinear spectroscopy, including the sum frequency generation (SFG) spectroscopy and second harmonic generation (SHG), has been shown to be a versatile in situ optical probe for exploring interfacial chemistry. Using the surface-selective SHG and SFG spectroscopy, we dedicate to elucidate the details of the molecular behaviors at surfaces or interfaces that play vital roles during the course of nanocatalysis, biological functions, polymer chemistry, photovoltatic conversions, and etc. We are also currently building a high-resolution broad-band SFG vibrational spectrometer with the subwavelength spectroscopic resolution and femtosecond time resolution. This new capability will enable us to study the energy flow as well as the subtle and dynamic chemical changes during the interfacial reactions.