Monodomain magnetic nanoparticles, due to their size, demonstrate physical properties not seen in the bulk materials, such as size-dependent magnetization reversal fields. They can be also made into a magnetic suspension or ferrofluid. There is thus growing interest in the application of these materials to ferrofluids, spintronics, directed assembly, as well as for imaging and therapeutic applications. In this article, we provide an overview of these materials, discuss the fundamental physical properties, describe several routes for the “bottom-up” generation of these materials, and identify major challenges for the future of these fields. The articles in this issue describe various aspects of the characterization and application of magnetic nanoparticles.