This is the first symposium to explicitly include nitrogen in discussions on nondopant light impurities in Si. Ordering of the impurity listing (oxygen, carbon, hydrogen and nitrogen) deserves examination. It is obvious that the impurities are not listed alphabetically nor are they listed in the order they appear in the periodic table. Presumably the ordering does not simply reflect a bias of the organizing chairmen. The impurities are, in fact, listed in an order of recognition on a chronological scale of their importance in Si technology. Equilibrium solid solubility for the different impurities, as well as concentrations of the impurities in the processing environment, probably determined the chronology. Based upon published literature, the importance of oxygen was recognized in the mid 50's, carbon in the mid 60's and hydrogen in the mid 70's. Now in the mid 80's nitrogen is receiving increased attention as an impurity in Si.