Magnetite (Fe3O4), which is an important member of spinel type ferrites, has been widely used in various industrial products such as recording materials because it has the excellent magnetic properties. Recently, bio-applications of Fe3O4 nanocrystals, e.g., magnetic resonance imaging, drug delivery, and hyperthermia, have attracted much attention because of the non-toxicity property and high chemical stability. Thus, the demand for Fe3O4 is increasing, and industrial synthesis methods which can meet the demand are needed. This book examines the synthesis of nanocrystals using solvent-free conventional and novel methods.