Progress in nanosynthesis has succeeded in making nanoscale particles from iron oxide under precise quality control. Given the recent great advances in polymer manufacturing, antibody purification, DNA/RNA synthesis and magnetic resonance imaging (MRI), such iron oxide nanoparticles (IONPs) have been enriched with many variables and attracted great interest in studying their potential biomedical applications. After nearly two decades' effort, IONPs have become a powerful platform in many diverse aspects of biomedicine, including MRI, gene and drug delivery, and hyperthermia. While some studies are still at the proof-of-concept stage, others have now been widely used in clinics. With the on-going efforts to enhance their targeting ability and endow more functions, IONPs' future applications are highly expected.