This work has involved the study of the magnetic behaviour of small magnetic nanoparticle systems. Due to the reduced size of magnetic nanoparticles they present distinctive properties, such as size and surface effects, that have been analysed in this work, as well as the effect of interactions in such systems. The samples chosen for the study were magnetite particles in the form of a ferrofluid and Co nanoclusters in a nonmagnetic matrix of Cu. Both systems present very narrow particle size distributions, which facilitates the interpretation of the data. The samples have been subjected to basic characterisation, which includes the determination of the distribution of magnetic particle sizes using the magnetisation curves at room temperatures, TEM microscopy and X-ray diffraction, in the case of the ferrofluid samples. For the nanoclusters, a time of flight spectrometer has been used to obtain the number of atoms per cluster. Many of the measurements have been performed at low temperatures, where thermal effects are minimised. For such measurements the samples have been frozen in a zero applied field, so that they have a random distribution of magnetic moments prior to the measurement. The energy barrier distributions have been calculated via the temperature decay of remanence (TDR). From this study, an effective anisotropy constant has been calculated. For the study of the interactions, surface and size effects, magnetisation, susceptibility (ZFC), remanence and delta-M curves, as well as the time dependence of magnetisation have been studied. The attempt frequency of the different particle size systems has been calculated using different techniques. The basic magnetic behaviour can be explained on the basis of the Neel blocking model. It has been found that the systems with the smaller particles have significant surface effects, which are enhanced at lower temperatures. Interactions, which are weak due to the low concentration of magnetic material in the samples (<10%), have been found to be overall demagnetising and the evolution of the magnetic properties with dilution has been explained. As is the case for the surface effects, interaction effects are stronger at low temperatures due the reduction of thermal effects. The experimental results have been compared with calculations from a Montecarlo model for fine particles, which includes the effects of concentration, anisotropy, particle size and temperature.