Abstract: This monograph presents an overview of friction stir processing (FSP) of surface metal-matrix composites (MMCs) using the AZ91 magnesium alloy. The reported results in relation to various reinforcing particles, including silicon carbide (SiC), alumina (Al₂O₃), quartz (SiO₂), boron carbide (B₄C), titanium carbide (TiC), carbon fiber, hydroxyapatite (HA), in-situ formed phases, and hybrid reinforcements are summarized. AZ91 composite fabricating methods based on FSP are explained, including groove filling (grooving), drilled hole filling, sandwich method, stir casting followed by FSP, and formation of in-situ particles. The effects of introducing second-phase particles and FSP process parameters (e.g., tool rotation rate, traverse speed, and the number of passes) on the microstructural modification, grain refinement, homogeneity in the distribution of particles, inhibition of grain growth, mechanical properties, strength–ductility trade-off, wear/tribological behavior, and corrosion resistance are discussed. Finally, useful suggestions for future work are proposed, including focusing on the superplasticity and superplastic forming, metal additive manufacturing processes based on friction stir engineering (such as additive friction stir deposition), direct FSP, stationary shoulder FSP, correlation of the dynamic recrystallization (DRX) grain size with the Zener–Hollomon parameter similar to hot deformation studies, process parameters (such as the particle volume fraction and external cooling), and common reinforcing phases such as zirconia (ZrO₂) and carbon nanotubes (CNTs).