This talk will present an overview of extrinsic and intrinsic stability issues of high-k dielectrics, such as ZrO@sub 2@, HfO@sub 2@, Y@sub 2@O@sub 3@, their alloys with SiO@sub 2@ or Al@sub 2@O@sub 3@, and their interfaces with electrodes and Si. While thermodynamics predicts that all high-k gate dielectrics currently under investigation are stable in contact with Si, interfacial reactions have been reported. We show that gate oxide stoichiometry and processing conditions, such as oxygen excess or reducing conditions, can explain reactions and are consistent with predictions from thermodynamics. Intrinsic stability issues include phase separation of silicates and aluminates, and will be discussed in the context of equilibrium as well as metastable phase diagrams, respectively. A combination of experimental methods is needed to experimentally analyze the stability of these ultrathin layers. We have used electron energy-loss spectroscopy (EELS), atomic resolution Z-contrast imaging, high-resolution transmission electron microscopy, small angle x-ray scattering (SAXS) and x-ray absorption spectroscopy fine structure analysis (XANES) to dielectric layers after high temperature anneals necessary for CMOS device processing. For example, phase separated microstructures of Hf-silicate films with different compositions show different morphologies and kinetics, due to mechanisms of microstructural evolution by nucleation and growth, and spinodal decomposition, respectively, consistent with the predictions from metastable phase diagrams. SAXS is used to study the kinetics of phase separation. Oxygen deficiency can lead to silicide reactions, whereas oxygen excess in the films is responsible for interfacial silicate reactions in rare earth oxide films on Si. This research was performed in collaboration with J.-P. Maria, A. Kingon, G. Parsons, P. Lysaght, P. C. McIntyre, S. Ramanathan, and T.P. Ma.