Magnetic colloids, or ferrofluids, have been studied to probe the fundamental size-dependent properties of magnetic particles and have been harnessed in a variety of applications. The magnetorheological properties of magnetic colloids have been exploited in high-performance bearings and seals. The deposition of magnetic dispersions on platters and tapes marked the earliest embodiments of magnet information storage. Magnetic particles enhance contrast in magnetic resonance imaging and promise future diagnostic and drug delivery applications. The need to explore the scaling limits of magnetic storage technology has motivated the preparation of size-tunable monodisperse magnetic nanoparticles with controlled internal structures. The study of these nanoparticles is critical to efforts to separate the role of defects from intrinsic, finite size effects.