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The nanostructures considered here are magnetic and characterized by structural length scales ranging from a few interatomic distances to about one micrometer. The basic length unit is the nanometer (1 nm = 10–3 μm = 10–9 m), corresponding to about four interatomic Fe-Fe distances. Magnetic nanostructures pose experimental challenges, exhibit interesting physical phenomena, and have many present or potential applications. An important aspect is that structural lengths affect, but only partly determine, the magnetic length scales encountered in the structures. Examples are domains in semihard nanoparticles, where both the domain size and the domain-wall thickness may be smaller than the particle size, and polycrystalline soft-magnetic nanostructures, where the magnetic correlation length is much larger than the crystallite size. Due to rapid progress in the fabrication and processing of nano structures, it is now possible to realize a broad variety of geometries, crystalline textures, and chemistries. For a given geometry, the structures can be fabricated using a variety of magnetic materials (compounds), with different local magnetic properties and crystalline textures.