Despite considerable progress in tumor treatment achieved over the last decades, cancer remains one of the leading causes of death worldwide. New treatment strategies based on magnetic nanoparticles have a great potential to significantly improve tumor therapy. In particular, superparamagnetic iron oxide nanoparticles (SPIONs) constitute a versatile platform for targeted drug delivery both as carriers of drugs and/or targeting molecules, as well as by offering the possibility of increased drug accumulation in the diseased area by external magnetic field. Moreover, these nanoparticles can be visualized by imaging modalities like magnetic resonance imaging (MRI), magnetorealxometry (MRX) or magnetic particle imaging (MPI), which allows direct qualitative and/or quantitative insight into the particle distribution in the tumor area. In this way, the control of drug targeting and an estimation of the therapeutic efficacy will be possible in the future.