Nowadays, the design of multifunctional nanoparticles are able to fulfill several requirements for a specific application is the basis of multidisciplinary approaches to the problem. In the case of biomedical applications, one of the main goals is to synthesize multifunctional magnetic nanoparticles that exhibit the highest saturation magnetization as possible and have surfaces properly functionalized that allow them to selectively attach to target cells or tissues. For a wide range of applications, the use biomaterial to modified characteristics of nanoparticles has attracted much attention. Although other materials can be found that fulfill more appropriately the magnetic requirements for biomedical applications, other concepts such as biocompatibility or toxicity must be taken into account. Iron oxides not only show interesting size-dependent magnetic properties and can be functionalized with both organic and inorganic compounds but also they are thought to be bio-compatible and non-toxic, which makes them excellent candidates for biomedical applications and in in-vivo experiments.