Because properties of colloids containing micro- and nano-particles are much influenced by the structure of, and interactions occurring at, the particle surface, it is important to be able to characterize the particle surface in situ and nondestructively. Nonlinear light scattering from colloidal particles has been developed into a powerful and versatile technique for characterizing particle surfaces since the first demonstration of the detection of second harmonic generation from molecules adsorbed on micrometer-sized colloidal particles by Eisenthal and co-workers (Wang, H.; Yan, E. C. Y.; Borguet, E.; Eisenthal, K. B. Second Harmonic Generation from the Surface of Centrosymmetric Particles in Bulk Solution. Chem. Phys. Lett. 1996, 259, 15-20). At present, second harmonic light scattering from the particle surface can be quantitatively described by theoretical models and used to measure the adsorption kinetics, molecular structure, and reaction rates at the surfaces of a variety of micrometer- to nanometer-sized particles, including biological cells.