Two types of nanostructures in SiC matrices are studied. Heterocrystalline structures are based on different polytypes, e.g. 3C-SiC and 4H-SiC. Multi quantum well structures with 3C wells embedded in 4H barriers show photoluminescence that can be interpreted in terms of a type-II heterostructure character and a built-in electric field due to the pyroelectricity of 4H. Germanium is incorporated in 4H-SiC matrices in form of nanometer-sized crystals. Their size, crystal structure and orientation depends on the preparation conditions. Ge nanocrystallites should give rise to absorption peaks below those of the matrix. The system Ge-4H-SiC represents a type-I heterostructure. A strong size- and structure-dependent luminescence from quantum states is predicted.