Magnetic particle imaging (MPI) is an attractive new modality for imaging distributions of iron oxide nanoparticle tracers in vivo. With exceptional contrast, high sensitivity, and good spatial resolution, MPI shows promise for clinical imaging in angiography and oncology. Critically, MPI requires high-quality iron oxide nanoparticle tracers with tailored magnetic and surface properties to achieve its full potential. In this review, we discuss optimizing iron oxide nanoparticles' physical, magnetic, and pharmacokinetic properties for MPI, highlighting results from our recent work in which we demonstrated tailored, biocompatible iron oxide nanoparticle tracers that provided two times better linear spatial resolution and five times better signal-to-noise ratio than Resovist.