Aqueous Mg batteries are promising power sources for different applications due to their remarkable high theoretical energy density, good voltage and low cost. In this chapter, we introduce conventional and newly proposed types of aqueous Mg batteries based on the cathode reactants as well as their working principles. Disadvantages and existing problems of water-based systems are summarized. Moreover, approaches for battery performance improvement and associated research progress are reviewed. Lastly, some outlooks concerning the applications, such as in maritime environments and future research focuses of Mg-based batteries are given. More effort regarding the development of novel Mg-based anodes, advanced catalysts and electrolyte additives is strongly recommended.