Magnesium(Mg) metal anodes are attractive for next-generation rechargeable cells due to their high volumetric capacity, low redox potential, elemental abundance, and intrinsic safety. Yet their reversibility is fundamentally constrained by mechanistic challenges distinct from monovalent metal anode counterparts. The divalent charge of Mg2+ induces strong solvation, leading to large desolvation barriers, while its strong reducibility drives parasitic electrolyte decomposition. These coupled effects yield ion-blocking passivation layers, hydrogen evolution, self-corrosion, and morphological instability of Mg metal anodes. Building on these mechanistic insights, this review provides a mechanism-driven perspective on Mg metal anodes: we delineate interfacial challenges in organic and aqueous electrolyte systems by dissecting the coupled roles of Mg2+ solvation–desolvation, parasitic interfacial reactions, Mg plating/stripping kinetics, and mechanical evolution; on this basis, we articulate cross-cutting design principles—encompassing electrolyte formulation, artificial interfacial layers, alloying strategies, and 3D host architectures—that balance suppression of parasitic pathways with efficient Mg2+ transport. Special attention is given to contrasting kinetic bottlenecks of organic electrolyte systems with thermodynamic constraints of aqueous media, and extracting unified design principles bridging these two regimes. Finally, we outline a co-design strategy across electrolytes, interfacial layers, and electrode architectures as a pathway toward reversible, scalable, and safe Mg metal cells. © 2026 Wiley-VCH GmbH.