The in vivo use of magnetic nanoparticles is attracting considerable interest as a means of delivering personalized medicine. Biocompatible nanoparticles that can be drawn toward a magnet are being investigated as site-specific drug delivery agents. Transfection of cells with nanosized particles observable by magnetic resonance imaging (MRI) offers a way to monitor experimental cell therapies. However, one size does not necessarily fit all. Realizing the clinical potential of these novel nanocarriers means finding the correct magnetic nanoparticle for each particular job.