Magnetic nanoparticles (MNPs) are promising materials for magnetic resonance imaging (MRI) contrast agents and drug delivery due to their unique magnetic properties and ability to function at the cellular and molecular level. Recent advances in nanotechnology have enabled precise engineering of MNP features for biomedical applications. MNPs with higher magnetic moments, non-fouling surfaces, and increased functionalities are being developed for applications in detecting, diagnosing, and treating malignant tumors, cardiovascular disease, and neurological disease. Functional ligands such as fluorophores and permeation enhancers enhance the applicability and efficacy of MNPs. MNPs can be tailored for multi-modal imaging, drug delivery, and combined therapeutic approaches. Superparamagnetic iron oxides (SPIOs) have been used as MRI contrast agents for over two decades, with several forms of ultrasmall SPIOs undergoing clinical trials. MNPs have been evaluated for targeted drug delivery through magnetic drug targeting (MDT) and active targeting via high affinity ligands. MNPs offer an attractive means of remotely directing therapeutic agents to disease sites while reducing side effects. MNPs have shown some success in early clinical trials, with pre-clinical studies indicating that improvements in design can overcome limitations of MDT technology. The use of MNPs as carriers in multifunctional nanoplatforms for real-time monitoring of drug delivery is an area of intense interest. Challenges in MNP application include their behavior in vivo, with their efficacy often compromised by recognition and clearance by the reticuloendothelial system (RES) and biological barriers. The physicochemical properties of MNPs affect their pharmacokinetics and biodistribution. Techniques such as reducing size and grafting non-fouling polymers improve their "stealthiness" and blood circulation time. Next-generation MNP-based MR imaging contrast agents and drug delivery carriers incorporate novel nanocrystalline cores, coating materials, and functional ligands to improve detection and delivery. New MNP cores such as doped iron oxide nanocrystals, metallic/alloy nanoparticles, and nanocomposites offer high magnetic moments. Surface coatings such as stable gold or silica shell structures allow for the application of otherwise toxic core materials. MNPs have been used for drug delivery, with studies showing their potential for targeted delivery of pharmaceuticals. MNPs have been evaluated for their ability to deliver drugs to cancer cells, with some showing cytotoxicity toward cancer cells. MNPs have also been used for imaging, with studies showing their potential for detecting tumors. MNPs have been used for passive and active targeting, with passive targeting exploiting structural abnormalities in the vasculature of tumors, while active targeting uses targeting molecules with high affinity for unique molecular signatures on malignant cells. MNPs have been used for intracellular delivery and controlled release, with studies showing their ability to deliver drugs to cell cytoplasm. MNPs have been used for biodistribution and clearance, with studies showing their long-term fate in vivo. MNPs have beenMagnetic nanoparticles (MNPs) are promising materials for magnetic resonance imaging (MRI) contrast agents and drug delivery due to their unique magnetic properties and ability to function at the cellular and molecular level. Recent advances in nanotechnology have enabled precise engineering of MNP features for biomedical applications. MNPs with higher magnetic moments, non-fouling surfaces, and increased functionalities are being developed for applications in detecting, diagnosing, and treating malignant tumors, cardiovascular disease, and neurological disease. Functional ligands such as fluorophores and permeation enhancers enhance the applicability and efficacy of MNPs. MNPs can be tailored for multi-modal imaging, drug delivery, and combined therapeutic approaches. Superparamagnetic iron oxides (SPIOs) have been used as MRI contrast agents for over two decades, with several forms of ultrasmall SPIOs undergoing clinical trials. MNPs have been evaluated for targeted drug delivery through magnetic drug targeting (MDT) and active targeting via high affinity ligands. MNPs offer an attractive means of remotely directing therapeutic agents to disease sites while reducing side effects. MNPs have shown some success in early clinical trials, with pre-clinical studies indicating that improvements in design can overcome limitations of MDT technology. The use of MNPs as carriers in multifunctional nanoplatforms for real-time monitoring of drug delivery is an area of intense interest. Challenges in MNP application include their behavior in vivo, with their efficacy often compromised by recognition and clearance by the reticuloendothelial system (RES) and biological barriers. The physicochemical properties of MNPs affect their pharmacokinetics and biodistribution. Techniques such as reducing size and grafting non-fouling polymers improve their "stealthiness" and blood circulation time. Next-generation MNP-based MR imaging contrast agents and drug delivery carriers incorporate novel nanocrystalline cores, coating materials, and functional ligands to improve detection and delivery. New MNP cores such as doped iron oxide nanocrystals, metallic/alloy nanoparticles, and nanocomposites offer high magnetic moments. Surface coatings such as stable gold or silica shell structures allow for the application of otherwise toxic core materials. MNPs have been used for drug delivery, with studies showing their potential for targeted delivery of pharmaceuticals. MNPs have been evaluated for their ability to deliver drugs to cancer cells, with some showing cytotoxicity toward cancer cells. MNPs have also been used for imaging, with studies showing their potential for detecting tumors. MNPs have been used for passive and active targeting, with passive targeting exploiting structural abnormalities in the vasculature of tumors, while active targeting uses targeting molecules with high affinity for unique molecular signatures on malignant cells. MNPs have been used for intracellular delivery and controlled release, with studies showing their ability to deliver drugs to cell cytoplasm. MNPs have been used for biodistribution and clearance, with studies showing their long-term fate in vivo. MNPs have been