Noninvasive cellular imaging allows the real-time tracking of grafted cells as well as the monitoring of their migration. Several techniques for in vivo cellular imaging are available that permit the characterization of transplanted cells in a living organism, including magnetic resonance imaging (MRI), bioluminescence, positron emission tomography, and multiple photon microscopy. All of these methods, based on different principles, provide distinctive, usually complementary information. In this review, we will focus on cell tracking using MRI, since MRI is noninvasive, clinically transferable, and displays good resolution, ranging from 50microm in animal experiments up to 300microm using whole body clinical scanners. In addition to information about grafted cells, MRI provides information about the surrounding tissue (i.e., lesion size, edema, inflammation), which may negatively affect graft survival or the functional recovery of the tissue. Transplanted cells are labeled with MR contrast agents in vitro prior to transplantation in order to visualize them in the host tissue. The chapter will focus on the use of superparamagnetic iron oxide nanoparticles (SPIO), because they have strong effects on T2 relaxation yet do not affect cell viability, and will provide an overview of different modifications of SPIO and their use in MR tracking in living organisms.