Ultrathin metal films have great potential for applications in areas such as magnetic sensors, recording materials, and novel devices such as spin filters or transistors. This research monograph discusses the close correlation between the magnetic and structural properties of thin films in the context of numerous examples of epitaxial metal films, while emphasis is laid on the stabilization of novel structures compared to the bulk material. Further options, possibilities, and limits for applications are given. Techniques for the characterization of thin films are addressed as well.