In the last few decades, superparamagnetic iron oxide nanoparticles (SPIONs—particularly magnetite (Fe3O4)/maghemite (Fe2O3) nanoparticles) have gained a great deal of attention in many biomedical applications, including magnetic targeting based cell isolation/sorting, tissue engineering, gene delivery, and magnetofection, due to their unique magnetic properties, excellent chemical stability, biodegradability, and low toxicity as compared to other magnetic materials (for instance, Co, Mn, and Ni). But recently, SPIONs (in the form of ferrofluids—i.e., SPIONs dispersed in a carrier fluid) have become a highly promising candidate for their use as therapeutic and diagnostic (theranostic) agents in cancer treatment applications such as magnetic fluid hyperthermia (MFH) and magnetic resonance imaging (MRI), respectively. However, the theranostic efficacies of the SPIONs (or ferrofluids) might alter due to the differences in their physicochemical/dispersibility/magnetic properties that are significantly impacted by their synthesis methods and their stabilization process. In this chapter, we have initially discussed the crystal structure/composition and different synthesis methods of the SPIONs. Then, we have described the role of the SPIONs in the formation of the ferrofluids along with their stabilization process via diverse interactions. Finally, we have discussed about their (1) intrinsic cancer theranostic applications of SPIONs such as magnetic fluid hyperthermia, magnetic resonance imaging, and magnetic nanoparticle-based drug delivery and (2) combined cancer theranostics applications including MRI as an adjuvant to fluorescence imaging, thermo-chemotherapy, thermo-radiotherapy, and thermo-immunotherapy. © Springer Nature Switzerland AG 2019. All rights reserved.