Ternary-chalcopyrite crystals contain a variety of point defects—the most common of which are vacancies, antisite ions, and impurities. Usually these defects are isolated, but they can also appear as complexes involving two or more of the simple defects. Depending on the material, the concentrations of these defects may vary from a few hundred parts per billion to a few hundred parts per million. Many of the point defects in the ternary chalcopyrites have associated optical-absorption bands with significant oscillator strengths. It is these absorption features that become important when the crystals are exposed to intense laser beams during device operation. Even a small amount of absorption will seriously degrade the performance of the device if any of the wavelengths of the various propagating beams happen to overlap an absorption band. This phenomenon can be a problem for both second-harmonic-generator and optical-parametric-oscillator applications. In general the absorption leads to heating of the crystal and results in-thermal lensing (due to temperature dependence of the index of refraction) and dephasing of the beams, and it can ultimately lead to thermal fracturing of the crystal. Thus it is important to develop a fundamental understanding of the defect structure of the ternary-chalcopyrite crystals if they are to serve as the critical component in midinfrared frequency-conversion devices. Once the nature and behavior of the point defects are established, processes can be developed to remove the defects from the crystals either during the growth itself or during post-growth treatments.