The rapidly developing material systems involving GaN and (Al, Ga, In)N must be thoroughly characterized in order for the field to advance in an efficient manner. Some of the more useful characterization techniques are: Hall effect, DLTS, and TSC, for electrical properties; photoluminescence and reflectance, for optical properties; x-ray and TEM, for structural properties; and SIMS for analytical properties. Also, theory is helpful in the identification of defects and impurities.