In this book is describe a new, low cost method forthe fabrication of high quality magnetic fluids forbiomedical applications. These fluids consist ofnanoparticles with different sizes stabilized bydifferent organic molecules dispersed in a carrierliquid. The surfactantmolecules stabilize the individual nanoparticlesagainst aggregation and render them easilydispersible in both polar protic and non-polarsolvents. Due to their small size,the nanoparticlesexhibit magnetic properties which can be reversiblyswitched on and off with an external magnetic fieldwhich makes them attractive as carriers in variousdrug delivery schemes. In addition, their highsurface reactivity allows the attachment of smallgold grains to their surfaces which facilitates thefunctionalization of the magnetic crystals withvarious bioactive molecules. The resultingnanoarchitectures are highly stable, biocompatibleand multifunctional opening up new opportunitiesfor the implementation of the magnetic nanoparticlesin the diagnosis and therapy of various diseases.