In this chapter, an update of recent advances in the synthesis of iron oxide magnetic nanoparticles (NPs), consisting of uniform magnetic cores stabilized in water forming biocompatible aqueous colloids, is presented. Aqueous synthetic routes granting NP shape control by the synthesis of antiferromagnetic precursors or highly ordered magnetic nanostructures by biomineralization are presented. Alternative synthetic routes in organic and polyols media allowing NP size and aggregation control and surface functionality tuning are also included. Finally, other routes such as microwave-assisted methods and electrochemistry or plasma techniques are discussed. Special attention has been paid to determine the reproducibility of the synthetic routes, the possibilities of large production and the mechanism of NP formation and self-assembling processes that could be a powerful tool to control the dimensions and morphology of NPs, essential for the optimization of their properties.