A principal application of nonlinear optics is frequency-converting laser outputs. Adding a frequency-converting crystal to an existing laser system is practical because lasers are widely available and, for the most part, reliable. However, the idealized plane-wave treatment needs some modifications to account for real-world laser sources. This chapter first looks at modifying the plane-wave equations so that they are applicable to Gaussian laser beams. Then, it introduces optical cavities to resonate one or more of the frequencies involved. In addition to looking at resonator cavities with the aim of increasing the efficiency, the chapter also introduces a type of device in common use called an optical parametric oscillator (OPO). For a Gaussian laser beam, the peak intensity at the focus is proportional to the total power in the beam. An interesting effect occurs with difference-frequency generation (DFG) in a resonant cavity.