This review focuses on the recent developments and strategies in the preparation, structure, and magnetic properties of naked and surface-functionalized iron oxide nanoparticles (NPs) and their applications. The particles must possess high magnetic saturation, stability, biocompatibility, and interactive functions at the surface to be practical. Surface modification can be achieved using organic or inorganic materials, such as polymers, biomolecules, silica, metals, and metal oxides. The challenges and future trends in the synthesis and surface functionalization of iron oxide NPs are discussed, along with the importance of protecting shells for stabilization and further functionalization. The review covers various synthesis methods, including co-precipitation, thermal decomposition, hydrothermal synthesis, microemulsion, and sonochemical synthesis, each with its advantages and disadvantages. Surface functionalization using organic materials, polymers, and biological molecules is also detailed, highlighting the importance of maintaining magnetic properties while enhancing biocompatibility and biodegradability. Inorganic materials, particularly silica, are widely used for their stability, biocompatibility, and ability to enhance the application scope of iron oxide NPs. The review concludes by discussing the potential of composite NPs with controlled structures and interface interactions for future technological applications.This review focuses on the recent developments and strategies in the preparation, structure, and magnetic properties of naked and surface-functionalized iron oxide nanoparticles (NPs) and their applications. The particles must possess high magnetic saturation, stability, biocompatibility, and interactive functions at the surface to be practical. Surface modification can be achieved using organic or inorganic materials, such as polymers, biomolecules, silica, metals, and metal oxides. The challenges and future trends in the synthesis and surface functionalization of iron oxide NPs are discussed, along with the importance of protecting shells for stabilization and further functionalization. The review covers various synthesis methods, including co-precipitation, thermal decomposition, hydrothermal synthesis, microemulsion, and sonochemical synthesis, each with its advantages and disadvantages. Surface functionalization using organic materials, polymers, and biological molecules is also detailed, highlighting the importance of maintaining magnetic properties while enhancing biocompatibility and biodegradability. Inorganic materials, particularly silica, are widely used for their stability, biocompatibility, and ability to enhance the application scope of iron oxide NPs. The review concludes by discussing the potential of composite NPs with controlled structures and interface interactions for future technological applications.