Magnetic nanoparticles have a wide range of applications, from various engineering devices such as micro-electro-mechanical systems and lubrication and sealing of bearings, to biomedical applications. This chapter reviews the basics of magnetism as relevant to magnetic solids and specifically superparamagnetic nanoparticles. For sufficiently small ferromagnetic nanoparticles, magnetization can randomly flip direction under the influence of temperature without any external magnetic field influence. The settling of magnetic nanoparticles in a magnetic field is hindered by thermal energy inducing Brownian motion. There are a number of applications where the synergy of magnetic nanoparticles’ high surface area, dispersivity related to their size, and magnetization makes them very attractive as surface catalysts in various environmental and biomedical applications. A rapid separation of nanoparticles occurred, suggesting a non-classical settling phenomenon induced by magnetic forces which favor particle aggregation and therefore faster settling. Magnetic particle imaging is a new tomographic imaging technique which measures the spatial distribution of superparamagnetic nanoparticles.