Magnetic nanocomposites and nanoparticle assemblies have gained significant interest for biomedical applications including MRI, hyperthermia and targeted drug delivery. Their efficacy depends on colloidal stability, optimal magnetic properties and appropriate size and surface chemistry for good biodistribution. The first part of this thesis focuses on the preparation of sub 20 nm magnetic nanoparticles (IONP) and the modification of their surface charge for subsequent encapsulation or adsorption to the polymer poly n-butyl cyanoacrylate (PACA) to produce IONP dilute assemblies. In addition to this, IONP-dense assemblies were prepared using established methods, and the effect of IONP density on the magnetic resonance properties of the assemblies was assessed. It was found that the NMR response was very sensitive to the IONP density and also to the structure of the assembly. This suggests potential routes to novel materials with tunable MRI properties. The second part of the thesis focuses on the assembly of IONP into dense assemblies (or clusters) in organic media using a novel in situ assembly technique based on reducing surfactant coverage. Particular attention was paid to the kinetics of the assembly process and the effect of adding surfactants and/or gold nanoparticles to the metastable IONP assemblies was also studied.