Gate dielectrics have been key to MOS transistor performance and, at the same time, their Achilles heel for reliability. The standard failure mechanisms -- breakdown, threshold voltage instability -- have not changed, but insulator thickness reduction and materials changes continually challenge our understanding. This lecture will describe new and improved physics understanding and statistical methods which are needed to enable us to anticipate the effects of scaling on reliability. Topics to be covered include: the electrical and physical limits to gate dielectric scaling, recent insights into NBTI (Negative Bias Temperature Instability) and hot carrier damage, physical models of dielectric breakdown, projection to use condition including Weibull and non-Weibull statistics, progressive breakdown, and implications for IC reliability.