We present our chemical synthetic approaches to monodisperse magnetic nanoparticles (Co and FePt) and nanoparticle superlattices. Advances of magnetic recording technology have driven the development of new magnetic nanoparticle-based media with uniformity in both particle size and particle magnetics. Self-assembly of magnetic nanoparticles may offer an easy way of fabricating such media. The key step for successful self-assembly approach is to use structurally stabilized magnetic nanoparticles as building blocks to form uniform nanoparticle arrays. We have found that steric repulsion from long chain hydrocarbon surfactants is effective in particle stabilization process. A combination of surfactants such as trialkylphosphine/oleic acid (for Co) and oleic acid/oleyl amine (for FePt) has been successfully employed to control particle growth, stabilize the particles, and protect them from oxidation. The particles can be prepared by metal salt reduction and metal carbonyl decomposition. By varying metal/surfactant or metal/metal ratio, both particle size (2-11nm) and alloy composition can be tuned. These monodisperse magnetic nanoparticles can self-organize into regularly arrayed magnetic superlattices. Microscopic studies of the assemblies have shown that the symmetry of these assemblies is dependent upon many factors including particle's size and shape. Thermal annealing is applied to adjust interparticle spacing of the superlattice assemblies and to control internal particle structure. Magnetic properties of these assemblies can be easily tuned from superparamagnetic to ferromagnetic. These well-controlled magnetic nanoparticle assemblies are of interest for future fabrication of nanoelectronic devices, and will have great potential for ultra-high density magnetic recording.