Given the remarkable progress that has recently been obtained in animal studies, the clinical use of stem and progenitor cells to correct or replace defective cell populations may soon become a reality. In order to develop effective cell therapies, the location and distribution of these cells must be determined in a non-invasive manner. Magnetic resonance (MR) tracking of magnetically labeled cells following transplantation or transfusion may fulfill this requirement. Indeed, a series of recent studies indicate that MRI cell tracking has great potential for further evaluation and optimization of cell therapy. Due to its biocompatibility and strong effects on T2(*) relaxation, iron oxide nanoparticles appear to be the contrast agent of choice, and several methods now exist to shuttle sufficient amount of these compounds into cells. Most of the tracking work has been carried out in disease models of the central nervous system, but, recently, the infarcted heart has also received attention. With its excellent spatial resolution and the ability to track labeled cells over prolonged periods of time, MR monitoring of cell therapy is likely to become an important technique in the foreseeable future.