This thesis examines how dipolar interparticle interactions affect the magnetic properties of self-assembled structures of magnetic nanoparticles. It begins by determining the properties of non-interacting nanoparticles from dispersion, enabling unbiased conclusions regarding the effects of interparticle interactions versus those of individual nanoparticles. The study focuses on four types of magnetic nanostructures: loosely packed nanospheres, monolayers of ordered nanocubes in square arrays, double layers of nanocubes with non-magnetic spacers, and three-dimensional colloidal crystals of nanocubes. Non-interacting properties are assessed using small-angle X-ray and neutron scattering on dilute dispersions, complemented by electron microscopy, X-ray diffraction, and vibrating sample magnetometry. The structure and magnetism of the assembled particles are analyzed using grazing-incidence small-angle scattering and reflectometry, comparing results with nanoparticle properties in dispersion and theoretical expectations of dipolar interactions. The superball form factor is introduced to better describe the scattering data of cubically shaped nanoparticles, accounting for deviations from perfect cube shapes. Additionally, an evaporation-driven self-assembly method is developed to create long-range ordered monolayers of oleic acid-ligated nanoparticles, which can be extended to double layers, facilitating a systematic study of inte
