The article discusses the use of second-harmonic generation (SHG) and sum-frequency generation (SFG) techniques for studying surfaces and interfaces. SHG is a versatile tool that can detect sub-monolayer coverage of adsorbed molecules and monitor surface dynamics with sub-picosecond time resolution. It is particularly useful for metal, semiconductor, liquid/solid, gas/liquid, and biological systems. SHG can provide information on molecular orientation, surface structure, and vibrational spectra. SFG, an extension of SHG, uses infrared-visible light to selectively detect molecular species through their vibrational transitions. Both techniques offer unique advantages in surface and interface studies, making them valuable tools in various scientific disciplines.The article discusses the use of second-harmonic generation (SHG) and sum-frequency generation (SFG) techniques for studying surfaces and interfaces. SHG is a versatile tool that can detect sub-monolayer coverage of adsorbed molecules and monitor surface dynamics with sub-picosecond time resolution. It is particularly useful for metal, semiconductor, liquid/solid, gas/liquid, and biological systems. SHG can provide information on molecular orientation, surface structure, and vibrational spectra. SFG, an extension of SHG, uses infrared-visible light to selectively detect molecular species through their vibrational transitions. Both techniques offer unique advantages in surface and interface studies, making them valuable tools in various scientific disciplines.