IN THIS LAST PART OF THE BOOK, we must address some issues that lie at the heart of our purpose as analytical chemists. Quality is not a new management revelation to the metals analyst, but has always been the essence of his task. It does take different forms, however, each with an underlying but seldom-voiced philosophy. In this chapter we will discuss one very important mechanism for bridging the gap between doubt and certainty: the application of certified reference materials to calibration and validation.