Recent progresses on magnetic nanostructures have made these systems a particularly interesting class of materials for both scientific investigations and technological applications, including information storage, sensors, biomedicine, catalysis and energy. Their unique properties arising from the low dimensionality can be finely tuned by playing with a number of parameters such as composition, shape, size, surface morphology, etc. In particular, during the last several years, a special attention has been devoted to the investigation of short and long-range magnetic interactions as a powerful tool to modulate or inducing novel properties. Collective magnetic states can be for example created among nanoparticles through long-range dipole-dipole interactions, which lead the nanoparticles to assembly into regular crystal lattices or in a completely random configuration when dispersed in a fluid. A plethora of phenomena including the oscillatory interlayer exchange coupling, the giant/tunnel magnetoresistance and the exchange bias effects, can be observed when different magnetic materials are in contact, possibly with the presence of a non magnetic spacer.