Particles and notably magnetic particles are used more and more extensively in bioanalytical processes today owing to their ability to considerably facilitate the development of protocols (e.g., easy rinsing and exchange of buffers) and allow miniaturization. Such protocols largely combine the advantages of heterogeneous and homogeneous assays. Applications of these particles, e.g., in protein or nucleic acids purification, immunoassays, or enzymatic microreactors, are developing rapidly, as reflected by the number of commercial kits that are regularly launched. The particles are also increasingly used in microfluidics, in which their potential for miniaturization is at its best. For optimal applications, however, and notably for the analysis and biomolecules and biological samples that are often complex, mastering the interactions between various moleculesparticularly biopolymers such as nucleic acids and proteins and the particle surface-and the grafting of active compounds onto the particles is critical and is indeed the major bottleneck to success. These surface interaction properties are responsible for the direct performance of the particles, such as load capacity and kinetics. Most often, however, the bottlenecks to development and key features for performance lie in mastering the surface properties of the grafted active compounds. These properties are responsible for the intrinsic performance, such as load capacity or reaction kinetics, and are also crucial for the control of spurious effects such as nonspecific adsorption or loss of colloidal stability. By providing an intermediate layer between the particle and its environment, that can be tailored both physically and chemically, polymer coatings offer a very powerful tool for controlling the surface properties of micro-and nanoparticles and for their biofunctionalization. Intense research is being performed in this area, stimulated by the strong potential applications mentioned above, and by the recent development of numerous new chemical routes that simplify the synthesis/ grafting of polymers, such as controlled radical polymerization methods or click chemistry. We provide here a general review of the field. We first give a general overview of the main features and properties of micro-and nanoparticles that are important for bioapplications, such as specific surface area, surface charge, zeta potential, and colloidal stability. We then summarize the different particle morphologies and the configurations that polymers can adopt on their surfaces, from a structural and physical point of view. We also describe the different strategies that can be used to cover particles with polymers, including physisorption, covalent grafting, or direct polymerization onto the particle. This is followed by a section dedicated to the different characterization methods that can be used to master the development of coating protocols, and to control the final properties. We end with a description of a few typical applications, in which the consequences and importance of th...