Suspensions of both dispersed and clustered iron oxide nanoparticles are of interest for biomedical applications as diagnostic and therapeutic agents due to their bio-compatibility and interesting magnetic and magnetic resonance properties. Their application is critically dependent on the fact that these properties are highly size-dependant. By developing NP synthesis and assembly methods, to provide improved size control at the two levels of organisation, NP suspensions with controlled emergent magnetic properties have been prepared. The first part of this thesis presents the size-controlled synthesis of magnetic nanoparticles whose surface is functionalised with a wide range of stabilising ligands. An investigation was undertaken to elucidate the effect of the stabilising ligands on the physical properties. The second part of the thesis presents a novel method for the reproducible assembly, in suspension, of NPs to form clusters. The method can be used to produce clusters of controlled size in large quantities; it is also shown that the kinetics of assembly can be easily controlled and that this influences the emergent magnetic properties. Arising from this observation a study was undertaken to determine the role of cluster architecture on the magnetic resonance properties of their aqueous suspensions.