Magnetic nanoparticles (MNPs) are non-invasive imaging agents used in magnetic resonance (MR) imaging. They have evolved from passive targeting to enable cellular-specific targeting, drug delivery, and multi-modal imaging. Proper design criteria, such as size, coating, and molecular functionalization, are essential for effective MNP performance in vivo. This review discusses the physicochemical properties and surface modifications of MNPs, including coatings and targeting ligand functionalizations that enhance their ability to manage biological barriers. The review also covers the chemistries used to modify MNP surfaces, focusing on optimizing ligand activity while maintaining favorable physicochemical properties. MNPs, including metallic, bimetallic, and superparamagnetic iron oxide nanoparticles (SPIONs), have been used in various biomedical applications such as magnetic separation, biosensors, in vivo imaging, drug delivery, tissue repair, and hyperthermia. SPIONs are particularly favored for their low toxicity and reactive surface that can be modified with biocompatible coatings and targeting, imaging, and therapeutic molecules. Current SPIONs are in early clinical trials or experimental stages, with several formulations approved for clinical use in medical imaging and therapy. In vivo, MNPs must overcome biological barriers such as the blood-brain barrier (BBB) and intracellular barriers. These barriers can restrict NP function by blocking movement, causing physical changes, or inducing a negative host response. NP physicochemical properties, including morphology, hydrodynamic size, charge, and surface properties, significantly influence their biodistribution, cellular uptake, and ability to manage biological barriers. Hydrodynamic size affects NP clearance and permeability, while NP shape influences biodistribution and blood circulation time. Surface properties, such as charge and hydrophobicity, affect interactions with the adaptive immune system, plasma proteins, and non-targeted cells. To enhance targeting specificity, NPs are modified with molecular targeting ligands, including small organic molecules, peptides, proteins, antibodies, and aptamers. These ligands can increase targeting specificity, producing contrast agents that specifically illuminate targeted tissue and drug carriers that do not interact with healthy tissue. Multivalency, the enhanced binding avidity phenomenon, can increase NP binding to target cells. NP shape also influences targeting abilities, with oblique-shaped particles showing greater cell binding affinity compared to spherical NPs. For drug delivery, NPs must carry and protect a significant drug payload, typically determined by the type of coating and method of loading. Multiple drugs can be loaded to overcome cellular drug resistance, but careful planning is required to accommodate different therapeutics. The release mechanism and rate of the therapeutic cargo should be modulated for optimal therapeutic efficiency. NPs can be directed to target tissues through passive, active, or magnetic targeting approaches. Magnetic targeting uses external magnetic systems to direct MNPs localization, but its effectiveness is limited to target tissues close to the body's surface. To ensure safety, the toxicity of individual components and NP as a whole mustMagnetic nanoparticles (MNPs) are non-invasive imaging agents used in magnetic resonance (MR) imaging. They have evolved from passive targeting to enable cellular-specific targeting, drug delivery, and multi-modal imaging. Proper design criteria, such as size, coating, and molecular functionalization, are essential for effective MNP performance in vivo. This review discusses the physicochemical properties and surface modifications of MNPs, including coatings and targeting ligand functionalizations that enhance their ability to manage biological barriers. The review also covers the chemistries used to modify MNP surfaces, focusing on optimizing ligand activity while maintaining favorable physicochemical properties. MNPs, including metallic, bimetallic, and superparamagnetic iron oxide nanoparticles (SPIONs), have been used in various biomedical applications such as magnetic separation, biosensors, in vivo imaging, drug delivery, tissue repair, and hyperthermia. SPIONs are particularly favored for their low toxicity and reactive surface that can be modified with biocompatible coatings and targeting, imaging, and therapeutic molecules. Current SPIONs are in early clinical trials or experimental stages, with several formulations approved for clinical use in medical imaging and therapy. In vivo, MNPs must overcome biological barriers such as the blood-brain barrier (BBB) and intracellular barriers. These barriers can restrict NP function by blocking movement, causing physical changes, or inducing a negative host response. NP physicochemical properties, including morphology, hydrodynamic size, charge, and surface properties, significantly influence their biodistribution, cellular uptake, and ability to manage biological barriers. Hydrodynamic size affects NP clearance and permeability, while NP shape influences biodistribution and blood circulation time. Surface properties, such as charge and hydrophobicity, affect interactions with the adaptive immune system, plasma proteins, and non-targeted cells. To enhance targeting specificity, NPs are modified with molecular targeting ligands, including small organic molecules, peptides, proteins, antibodies, and aptamers. These ligands can increase targeting specificity, producing contrast agents that specifically illuminate targeted tissue and drug carriers that do not interact with healthy tissue. Multivalency, the enhanced binding avidity phenomenon, can increase NP binding to target cells. NP shape also influences targeting abilities, with oblique-shaped particles showing greater cell binding affinity compared to spherical NPs. For drug delivery, NPs must carry and protect a significant drug payload, typically determined by the type of coating and method of loading. Multiple drugs can be loaded to overcome cellular drug resistance, but careful planning is required to accommodate different therapeutics. The release mechanism and rate of the therapeutic cargo should be modulated for optimal therapeutic efficiency. NPs can be directed to target tissues through passive, active, or magnetic targeting approaches. Magnetic targeting uses external magnetic systems to direct MNPs localization, but its effectiveness is limited to target tissues close to the body's surface. To ensure safety, the toxicity of individual components and NP as a whole must