Medium energy ion scattering (MEIS) is used to examine high-K gate dielectrics (HfO2, Al2O3, silicates, metal oxynitrides...) and their interfaces on Si, Ge and GaAs. Critical materials changes occur at the interfaces during growth and processing at elevated temperature that are readily observed by MEIS. We discuss the changes, including the decomposition of the films, recrystallization, as well as oxide and silicate interface growth. The native oxides of Ge and GaAs are less stable than those of Si making capacitance control during growth and processing less serious on these alternative semiconductors. In addition, x-ray photoemission (XPS), electron microscopy (TEM), inverse photoemission (IPES), and synchrotron-based studies were performed on a range of high-K samples. The thickness, layered structure, and crystal phase of the as-deposited and annealed films have been studied as a function of growth conditions. Related work involves a determination of band structure and alignment in high-K gate stacks. Our experimental results on band alignment and phase are complemented by first-principles DFT calculations to study the properties of the different crystalline structures of HfO2 and ZrO2. It is found that the band gap, barrier height and dielectric response of these two materials are phase-dependent. Finally, as charge within the dielectric and at the interface remains a critical issue in high-K integration, we briefly discuss correlations between electrical properties (such as defects), composition and band alignment.