Polymersomes are entities formed from the self-assembly of amphiphilic polymers containing a large hydrophobic part. Similarly to liposomes, these entities present a hydrophilic core, surrounded by a hydrophobic bilayer. Such configuration exhibits great potential for biomedical applications by offering the possibility to encapsulate hydrophobic or hydrophilic structures. More specifically, the encapsulation of magnetic nanoparticles is of great interest. Such magneto-nanoassemblies exhibit high transverse-to-longitudinal relaxivity ratios and are therefore emerging as highly efficient T ₂ contrast agents. In addition to MRI applications, the use of appropriate magnetic particles (i.e. magnetothermal responsive), in combination with thermoresponsive polymers, opens the doors to smart drug delivery applications. After some generalities concerning polymersomes, their preparation and their potency in the biomedical field, this chapter will focus on recent researches about magneto-nanoassemblies and their characterization.