This chapter summarizes the recent developments in structures, properties, and applications of superparamagnetic iron oxide nanoparticles (SPIONs). It aims to explore how superparamagnetic iron oxide nanoparticles (SPIONs) can significantly improve the quality of magnetic resonance imaging (MRI) by acting as effective contrast agents, thereby aiding in the early diagnosis and monitoring of tumors. It discusses the application of SPIONs in magnetofection, where their magnetic properties are utilized to enhance the efficiency of gene delivery into target cells, potentially revolutionizing gene therapy and diagnosis. The chapter highlights the use of SPIONs in magnetic separation techniques, emphasizing their ability to quickly and efficiently isolate specific cell populations from complex mixtures, which is crucial for biomedical research and clinical diagnostics. It addresses the role of SPIONs in cell labeling for in vivo studies, enabling researchers to monitor cell behavior and migration using MRI, thus providing insights into cellular processes relevant to disease progression and treatment efficacy via imaging and diagnosis.