High-resolution electron energy-loss spectroscopy (HR-EELS) and scanning transmission electron microscopy (STEM) have been applied to investigate BaTiO3/SrTiO3 ferroelectric multilayers and high-κ gate dielectric Y2O3/Si(001). The experiments are mainly concentrated on interface reactions and interface defects. The preparation of the STEM specimens and various experimental parameters which have strong effects on the acquisition of electron energy-loss spectra from the interface are discussed. In BaTiO3/SrTiO3 multilayers, oxygen vacancies have been found to preferably aggregate at the rough upper interface (SrTiO3/BaTiO3) while the lower interface (BaTiO3/SrTiO3) is nearly defect free. This finding has been explained in terms of misfit strain and oxygen vacancy ordering. The space charges formed by the aggregation of oxygen vacancies decrease the dielectric constant of ferroelectric multilayers. The crystal-field splitting of the Ti L23 edges is revealed to sensitively reflect the misfit strains. The lattice parameters of the BaTiO3 thin layer have been determined by selected-area electron diffraction, which confirm the presence of the strong misfit strains. In the high-κ gate dielectric Y2O3/Si(001), the interfacial SiOx (1x2) layer and yttrium silicates formed by interface reactions between the deposited Y2O3 film and the silicon substrate have been investigated by HR-EELS. The interfacial SiOx layer is nearly pure amorphous SiO2, which is attributed to the silicon oxidation at the initial stage of the deposition. Yttrium silicates are revealed to form at the Y2O3/SiO2 interface. The formation of yttrium silicates is interpreted by direct chemical reactions between the deposited Y2O3 film and the interfacial SiO2. Additionally, electron beam induced diffusion of SiO2 is found to form yttrium silicates. According to the results of EELS and HRTEM, the possible interpretation is given in this work. The formation of yttrium silicates has been confirmed by the full multiple-scattering (FMS) calculations. The orientation relationships between the Y2O3 film and the silicon substrate are interpreted in terms of surface free energy and lattice match. The band-structure and full multiple-scattering methods have been combined to interpret the electron energy-loss near-edge structures (ELNES) of the ceramics involved in this thesis, including SiO2, TiO2, SrTiO3, BaTiO3, Y2O3, Y2SiO5, and Y2Si2O7. The ligand-field atomic multiplet method is also briefly discussed to interpret the white-line structures in the transition-metal compounds. The theoretical calculations have been concentrated on the Ti L23, Y M45, Si L23 edges, and especially the O K edge. In comparison with the measured electron energy-loss (EEL) spectra, the main features of these core-loss edges have been reproduced fairly well by the Ab-Initio calculations. The combination of these different Ab-Initio methods gives the clear relationships between the EEL fine structures and the atomic microstructures around the absorbing atom. The theoretical calculations reveal that the low-energy features of the EEL core-loss spectrum are dominated by cation-oxygen interactions in a short range, and the high-energy features are interpreted by oxygen-oxygen interactions in a relative long range.