Magnetism is one of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field. Materials may be classified by their response to externally applied magnetic fields as diamagnetic, paramagnetic, or ferromagnetic. Magnetic particles of nanometre size are attracting growing fundamental and technological interest because of their unique magnetic properties, which are dominated by superparamagnetism. This book presents a broad range of topics in this growing field, including their use as magnetic agents, receptor particles for microwaves as well as in shielding layers.