Sum frequency generation spectroscopy (SFG) is a nonlinear laser spectroscopy technique used to analyze surfaces and interfaces. It can be expressed as a sum of a series of Lorentz oscillators. In a typical SFG setup, two laser beams mix at an interface and generate an output beam with a frequency equal to the sum of the two input frequencies, traveling in a direction allegedly given by the sum of the incident beams' wavevectors. The technique was developed in 1987 by Yuen-Ron Shen and his students as an extension of second harmonic generation spectroscopy and rapidly applied to deduce the composition, orientation distributions, and structural information of molecules at gas–solid, gas–liquid and liquid–solid interfaces.  Soon after its invention, Philippe Guyot-Sionnest extended the technique to obtain the first measurements of electronic and vibrational dynamics at surfaces.  SFG has advantages in its ability to be monolayer surface sensitive, ability to be performed in situ (for example aqueous surfaces and in gases), and its capability to provide ultrafast time resolution. SFG gives information complementary to infrared and Raman spectroscopy.