In the development of Cu-based metallization for future ULSI devices, a number of metallurgical issues have to be addressed. In this paper, the results of an investigation on interfacial reactions of copper thin films on silicon, growth of epitaxial metal layer with epitaxial Cu layer as a seed on silicon, growht of epitaxial metal layer with epitaxial Cu layer as a seed on silicon, formation of metastable metal layer with epitaxial Cu layer as a seed on silicon, formation of metastable phases in multilayered metal/Cu systems, room temperature oxidation of silicon in the presence of $Cu_3Si$, epitaxial growht of Cu layer on single-crystal silicide film, diffusion barrier properties of TiW and TiW(films between Cu and silicon will be presented.