The ATLAS Muon Spectrometer will be extensively upgraded during the Phase-II program in Long Shutdown 3 to meet the requirements for operation at the High-Luminosity LHC, which will be characterized by higher luminosity, increased trigger rates, and elevated radiation levels. New Resistive Plate Chamber (RPC) detectors with 1 mm gas gaps will be installed in the inner barrel layer to improve trigger robustness and acceptance. The inner barrel Monitored Drift Tube (MDT) chambers will be replaced by small-diameter MDT chambers to optimize the performance in highoccupancy regions and to create space for the new RPC chambers. In addition, MDT chambers will be integrated into the Level-0 trigger to enhance the transverse momentum resolution and, consequently, the trigger selectivity. In the barrel–endcap transition region, new Thin Gap Chamber (TGC) triplet chambers will replace the existing doublets to reduce trigger rates from random coincidences. Most of the readout and trigger electronics of the above-cited detectors will be replaced. Finally, the power distribution systems of all three detector technologies will also be upgraded to mitigate ageing, obsolescence, and radiation effects. This contribution summarizes the upgrade strategy, the current project status, and results from prototype and production studies.