We review the epitaxy of Mn-based magnetic thin films and III-V semiconductors. It is demonstrated chat by exploiting the epitaxial relationship to GaAs, novel artificial magnetic films can be fabricated. Metastable magnetic tau MnAl, forced bcc Co crystal structures and thin films with controlled magnetic anisotropy are feasible due to coherent growth on the semiconductor substrate. Novel magnetic superlattices and nano-scale magnetic particles embedded in a semiconductor matrix are described. The future synergism between semiconductor and magnetic thin films in research and applications is briefly discussed.