Magnetic nanoparticles and their assemblies are subjects of considerable scientific interest for basic research, but also for applications as contrast agents in magnetic resonance imaging (MRI) and for hyperthermia. Such applications depend on the production of stable suspensions of the particles, it is important therefore to characterise the particles in suspension. In this work photon correlation spectroscopy was used to measure of the hydrodynamic size of the particles. NMR techniques were used to determine the stability and to quantify the contrast efficiency (relaxivity) of the suspensions. This work has also provided insight into the nature of the nanoclusters in suspension and into the mechanisms of their growth. In the first part of this thesis the synthesis, stabilisation and magnetic properties of aqueous magnetite nanocomposite suspensions which are formed in the presence of fatty acids or DNA are presented. For fatty-acid stabilised nanocomposites the NMR response is sensitively dependent on the method of preparation, which cab result in magnetically blocked or superparamagnetic nanoclusters. In the case of the DNA nanocomposites, the biomolecule acts as a template for the preparation of low dimensional assemblies, or magnetic nanowires, whose suspensions exhibit high relaxivity at low magnetic field. In this second part the synthesis, stabilisation and magnetic properties of magnetite nanoparticle suspensions formed in organic solvents in the presence of long chain surfactants are presented. The influence of nanoparticle size on the magnetic properties is discussed in detail. The NMR response of the particles in non-aqueous suspension is shown to conform to a model previously developed for aqueous suspensions of magnetite. Studies of the controlled clustering of the nanoparticles in organic solvents are presented. The mechanism and kinetics of nanocluster growth are discussed.