Magnetic nanoparticles are attractive targets owing to their unique characteristics that are not shared by bulk materials. Magnetic particles, ranging from nanometer-sized to 1 microm in size, are being used in an increasing number of medical applications. The important properties of magnetic particles for medical applications are nontoxicity, biocompatiblilty, injectability and high-level accumulation in the target tissue or organ. Magnetic nanoparticles modified with organic molecules have been widely used for biotechnological and biomedical applications as their properties can be magnetically controlled by applying an external magnetic field. They offer high potential for numerous biomedical applications, such as cell separation, automated DNA extraction, gene targeting, drug delivery, MRI and hyperthermia. When coated with, for example, an antibody, they can be applied in highly sensitive immunoassays or small substance recoveries. Furthermore, a novel application of magnetic nanoparticles and magnetic forces for tissue engineering, termed 'magnetic force-based tissue engineering' has been proposed. Particular attention had been paid to the preparation methods that allow the synthesis of particles of nearly uniform size and shape.