Tremendous progress has been made in the field of magnetic materials research and technology over the past few years. Superior properties and novel scientific questions arise due to our ability to either synthesize artificial structures or to tailor microstructures at the appropriate length scales. Microstructure, generally defined as the morphological arrangement of crystallites with similar or different phase constitutions and of the crystal defects, is controlled by the processing, which in turn affects the phases present, their topology and their dispersion. There are two key length scales that determine the magnetic behavior of materials. One is the characteristic length scale associated with the dimensional characteristics of the magnetic phenomenon under consideration. The second is the size of the microstructural feature of interest - grain size, shape, isolation, interfaces, inter-granular phases etc. It is important to identify the range where these two length scales overlap, for it is there that novel properties and phenomenon are usually observed.