In this lecture, some interesting and important new results on the relationship between crystalline defects and optical absorption in CdGeAs2 single crystals will be reviewed. Such crystals are very promising for infrared second harmonic generation, but their use has been limited by non-ideal transparency near 5.5 mu m. Absorption and other crystal properties were found to vary from one laboratory to another, from boule to boule and also within each boule (the latter showing both axial and radial variations). Previous absorption studies were hindered by the lack of high-quality single crystals. Use of the horizontal gradient freeze method together with a low thermal gradient now makes it possible to prepare better quality material and perform more systematic property studies. The role played by point defects (arising from non-stoichiometry and impurities), dislocations, faceting and growth orientation on IR optical absorption will be discussed. (c) 2006 Elsevier B.V. All rights reserved.