Although composites with, e.g.nanosized SiO₂, TiO₂, carbon black or gold are known for a long time, an increasing number of polymer composites comprising inorganic nanoparticles have been described only in the last 15 years. Frequently employed inorganic materials include metals (e.g. gold, silver or copper), semiconductors (e.g. PbS or CdS) or clay minerals (e.g.montmorillonite or vermiculite). In most cases, nanocomposites with spheric or plate like particles have been prepared so far but materials with nanorods (rod like particles, including nanotubes) also attracted attention. The structure of nanocomposites is essentially established by the arrangement of the particles in the polymer matrix. The particles may be dispersed as individual primary particles or as agglomerated particles (secondary particles), and the primary or secondary particles can be arranged randomly or in an ordered or oriented state, depending on the method of nanocomposite preparation and processing (including, e.g. coprecipitation, coevaporation, spin coating or drawing). In order to avoid agglomeration, the nanoparticles are frequently synthesised in situ or applied as surface modified particles, where a spheric inorganic core is preferentially surrounded by a shell composed of organic molecules (core–shell particles). Surface modification is applied routinely for clays, which is of importance for the degree of delamination of the individual silicate layers in the matrix (intercalation or exfoliation). Materials properties of nanocomposites, such as optical transparency, colour (including dichroism), iridescence, catalytic activity or superparamagnetism, can be favourable when compared with analogous composites with larger particles, and these properties may also be influenced by the structure of nanocomposites. The properties of nanocomposites are considered to provide applications in the areas of, e.g. optics, catalysis, electronics, magnetism or mechanics.