Iron oxide nanoparticles (NPs) have attracted significant attention due to their unique properties such as superparamagnetism, high surface-to-volume ratio, and ease of separation. This review summarizes methods for synthesizing iron oxide NPs, controlling their size and morphology, and their magnetic properties, along with recent applications in biomedicine, agriculture, and environmental science. Iron oxide NPs, particularly magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃), are biocompatible and can be functionalized with organic or inorganic molecules like surfactants, proteins, and enzymes to enhance their applications. They can be directed to specific organs or tissues using external magnetic fields for hyperthermia treatments. The synthesis of iron oxide NPs involves various methods including chemical, physical, and biological approaches, with chemical methods being the most commonly used due to their cost-effectiveness and high yield. Factors such as pH, ionic strength, and the ratio of Fe²⁺ to Fe³⁺ influence the size and shape of the NPs. Surface modification is crucial for improving biocompatibility and stability. Iron oxide NPs have diverse applications in medicine, including MRI, drug delivery, and cancer treatment, as well as in environmental applications like water purification. However, challenges remain in terms of toxicity, biocompatibility, and efficient large-scale production. The review highlights the importance of further research to optimize the properties and applications of iron oxide NPs for biomedical and industrial uses.Iron oxide nanoparticles (NPs) have attracted significant attention due to their unique properties such as superparamagnetism, high surface-to-volume ratio, and ease of separation. This review summarizes methods for synthesizing iron oxide NPs, controlling their size and morphology, and their magnetic properties, along with recent applications in biomedicine, agriculture, and environmental science. Iron oxide NPs, particularly magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃), are biocompatible and can be functionalized with organic or inorganic molecules like surfactants, proteins, and enzymes to enhance their applications. They can be directed to specific organs or tissues using external magnetic fields for hyperthermia treatments. The synthesis of iron oxide NPs involves various methods including chemical, physical, and biological approaches, with chemical methods being the most commonly used due to their cost-effectiveness and high yield. Factors such as pH, ionic strength, and the ratio of Fe²⁺ to Fe³⁺ influence the size and shape of the NPs. Surface modification is crucial for improving biocompatibility and stability. Iron oxide NPs have diverse applications in medicine, including MRI, drug delivery, and cancer treatment, as well as in environmental applications like water purification. However, challenges remain in terms of toxicity, biocompatibility, and efficient large-scale production. The review highlights the importance of further research to optimize the properties and applications of iron oxide NPs for biomedical and industrial uses.