The article reviews the use of engineered magnetic nanoparticles (MNPs) in medicine, highlighting their applications in magnetic resonance imaging (MRI), guided drug and gene delivery, magnetic hyperthermia cancer therapy, tissue engineering, cell tracking, and bioseparation. MNPs are composed of magnetic elements like iron, nickel, and cobalt, and their unique properties allow them to be manipulated by external magnetic fields. The review discusses the role of MNP size, composition, and surface chemistry in intracellular uptake, biodistribution, macrophage recognition, and cytotoxicity. It also addresses the challenges and strategies for optimizing MNP performance and minimizing toxicity, including the activation of oxidative stress and the importance of macrophage clearance. The article emphasizes the need for comprehensive studies on MNP toxicity and the development of nanotoxicology to ensure safe and effective biomedical applications.The article reviews the use of engineered magnetic nanoparticles (MNPs) in medicine, highlighting their applications in magnetic resonance imaging (MRI), guided drug and gene delivery, magnetic hyperthermia cancer therapy, tissue engineering, cell tracking, and bioseparation. MNPs are composed of magnetic elements like iron, nickel, and cobalt, and their unique properties allow them to be manipulated by external magnetic fields. The review discusses the role of MNP size, composition, and surface chemistry in intracellular uptake, biodistribution, macrophage recognition, and cytotoxicity. It also addresses the challenges and strategies for optimizing MNP performance and minimizing toxicity, including the activation of oxidative stress and the importance of macrophage clearance. The article emphasizes the need for comprehensive studies on MNP toxicity and the development of nanotoxicology to ensure safe and effective biomedical applications.