Materials interfaces such as grain boundaries or interfaces between dissimilar materials seriously affect the properties or performance of materials or devices, and  heterogeneous or nano-size effects associated with such interfaces frequently generate novel phenomena.  Our research group is engaged in basic researches on the structure and properties of materials interfaces applied to energy or environment technologies.  In this talk, we present our first-principles calculations of materials interfaces.  We deal with grain boundaries in SiC and Al, metal/SiC interfaces for power devices, Au/TiO ₂ interfaces as gold catalysts, and metal/Al₂O₃ interfaces for thermal barrier coatings.  We deal with mechanical properties such as ideal tensile strength and impurity-induced embrittlement, electronic properties such as Schottky-barrier heights, and catalytic properties, through the analysis of atomic and electronic structures.  We discuss the interface design through elucidating key factors such as “interface stoichiometry” dominating the interface properties.