After introducing linear optics from the perspective of polarizability and linear susceptibility, the authors cover incoherent second-harmonic generation. They then deal with the study of surfaces and interfaces, exploiting the intrinsic surface sensitivity of second-harmonic generation and sum-frequency generation. The final chapter discusses second-order imaging techniques, including confocal microscopy and two-photon excited fluorescence microscopy. Accessible to a wide range of scientists, this concise book stresses the reliability of nonlinear optical processes for probing surfaces and interfaces. Drawing on the insight offered in the text, scientists from many disciplines can now clearly understand and use second-order nonlinear optical methods.