The paper presents aspects of experimental observation and analysis of interaction effects in polydisperse magnetic fluids (suspensions of magnetic nanoparticles coated with surfactants) by small-angle scattering of synchrotron radiation and neutrons. The different modes of interaction are considered within the framework of the theory of dipolar liquids together with corresponding scattering structure-factors depending on the concentration of nanoparticles and the magnitude of their magnetic moments, including the effects of external magnetic fields.