Rapid advances in the synthesis of superparamagnetic nanoparticles have stimulated widespread interest in their use as contrast agents for visualizing biological processes with magnetic resonance imaging (MRI). With this approach, strong particle magnetism alters the MRI signal from nearby water protons and this, in turn, affects observed image contrast. Magnetic particle detection with MRI is therefore indirect and suffers from several associated problems, including poor quantification and tissue-dependent performance. Magnetic particle imaging (MPI) overcomes these problems by directly measuring the amount of superparamagnetic material at each location. Mass sensitivity, spatial resolution, and imaging time are also comparable to or better than those achieved with MRI. Moreover, MPI is relatively inexpensive, meets all current safety guidelines, is quantitative, provides unambiguous contrast with tissue-independent performance, and can detect lower particle concentrations. In this article, the basic principles behind MPI are described, factors affecting sensitivity and resolution are discussed, and potential utility for biomedical use is examined.