The chapter discusses the preparation, physical properties, and applications of magnetic nanoparticles (MNPs) in biomedicine. It highlights the importance of well-defined synthetic strategies for generating MNPs with uniform particle formation mechanisms and monodisperse samples. The text covers various methodologies for functionalizing MNPs and their characterization techniques, emphasizing the theoretical aspects of nanoparticle magnetism. Key topics include the advantages of using MNPs in biomedical applications, such as their superparamagnetic behavior at room temperature and colloidal stability in physiological environments. The chapter also explores the industrial applications of MNPs, particularly in magnetic recording media and biotechnology. Additionally, it delves into the environmental, industrial, biological, and analytical applications of MNPs, including their use in catalysis, remediation of contaminants, and diagnostic tools like MRI imaging. The preparation methods, physical properties, and size selection techniques for MNPs are discussed in detail, along with their impact on magnetic behavior and structure.The chapter discusses the preparation, physical properties, and applications of magnetic nanoparticles (MNPs) in biomedicine. It highlights the importance of well-defined synthetic strategies for generating MNPs with uniform particle formation mechanisms and monodisperse samples. The text covers various methodologies for functionalizing MNPs and their characterization techniques, emphasizing the theoretical aspects of nanoparticle magnetism. Key topics include the advantages of using MNPs in biomedical applications, such as their superparamagnetic behavior at room temperature and colloidal stability in physiological environments. The chapter also explores the industrial applications of MNPs, particularly in magnetic recording media and biotechnology. Additionally, it delves into the environmental, industrial, biological, and analytical applications of MNPs, including their use in catalysis, remediation of contaminants, and diagnostic tools like MRI imaging. The preparation methods, physical properties, and size selection techniques for MNPs are discussed in detail, along with their impact on magnetic behavior and structure.