This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. Applications for nonlinear optical devices were readily apparent after the underlying nonlinear effects were first demonstrated. Another class of nonlinear optics applications called nonlinear spectroscopy has established an understanding of molecular materials at the quantum level. Nonlinear absorption and scattering methods provide a means to identify trace quantities of unknown material constituents with high fidelity. Nonlinear effects in this book involve energy exchange between independent optical inputs, nonlinear phase shifts to optical fields, and energy exchange between optical fields and a material. The equations used in many studies of nonlinear optics are extended to incorporate dispersion and diffraction. The techniques contained in this book developed in discussing them are a good starting point for studying other, perhaps more exotic, nonlinear processes. Nonlinear interactions are described using a variety of systems of units in the literature.