This chapter explores the role of nanoscale reinforcements in enhancing the properties of magnesium-based composites for a wide range of applications, including structural, aerospace, automotive, and biomedical fields. It highlights the effects of ceramic particles (oxides, carbides, nitrides, and borides), carbon-based reinforcements (carbon nanotubes and graphene), calcium phosphate derivatives (for biodegradable implants), and metallic nanoparticles (e.g., Cu, Al, Ti, and Ni). Key mechanisms such as grain refinement, precipitation hardening, and bioactivity improvements are discussed in detail, along with challenges such as achieving uniform particle dispersion and strong interfacial bonding. Readers will gain insights into advancements in fabrication techniques, the unique benefits of each reinforcement type, and the future outlook for multifunctional magnesium nanocomposites.