Just how perfect can a perfect crystal be?

This overview addresses certain aspects of the perfection of the current ge neration of commercially manufactured monocrystalline silicon, specifically lattice uniformity and the more complex issues of interstitials, vacancies , and chemical impurities. The application areas considered are gamma-ray s pectroscopy, and the x-ray/crystal density approach to determination of the Avogadro constant. Some current imperfections, for which we show evidence, appear addressable by technical improvements in the production process, ha ve a much larger effect than is expected from intrinsic limitations emergin g from recent model calculations.