The growth of epitaxial rare-earth-transition-metal thin films is reported by magnetron sputtering on single-crystal MgO substrates. The use of epitaxial W buffer layers demonstrates a general approach to control the phase and orientation of the films. Structure and magnetism results for SmFe12(001) on W(100) and magnetically hard Sm2Co7 (110) and (001) on W(100) and (110), respectively, are highlighted to illustrate the utility of the approach.