Second-harmonic generation (SHG) and sum-frequency generation (SFG) are powerful techniques for studying surfaces and interfaces. SHG is a nonlinear optical process that occurs at interfaces where inversion symmetry is broken, making it highly surface-specific. It can detect sub-monolayer molecular adsorption and monitor surface dynamics with sub-picosecond time resolution. SHG is sensitive to surface structure and can provide information about molecular orientation and surface composition. It is particularly useful for studying metal, semiconductor, and liquid/solid interfaces. SFG, a related technique, allows for surface vibrational spectroscopy by using tunable lasers to excite vibrational transitions, enabling the selective detection of molecules based on their characteristic vibrational modes. Both techniques have been applied to various fields, including electrochemistry, biology, and materials science, offering unique advantages over conventional surface probes. SHG and SFG are versatile tools for surface studies, capable of providing detailed information about surface properties and dynamics across a wide range of interfaces.Second-harmonic generation (SHG) and sum-frequency generation (SFG) are powerful techniques for studying surfaces and interfaces. SHG is a nonlinear optical process that occurs at interfaces where inversion symmetry is broken, making it highly surface-specific. It can detect sub-monolayer molecular adsorption and monitor surface dynamics with sub-picosecond time resolution. SHG is sensitive to surface structure and can provide information about molecular orientation and surface composition. It is particularly useful for studying metal, semiconductor, and liquid/solid interfaces. SFG, a related technique, allows for surface vibrational spectroscopy by using tunable lasers to excite vibrational transitions, enabling the selective detection of molecules based on their characteristic vibrational modes. Both techniques have been applied to various fields, including electrochemistry, biology, and materials science, offering unique advantages over conventional surface probes. SHG and SFG are versatile tools for surface studies, capable of providing detailed information about surface properties and dynamics across a wide range of interfaces.