Abstract: Artificially controlling the solid-state precipitation in aluminum (Al) alloys is an efficient way to achieve well-performed properties, and the microalloying strategy is the most frequently adopted method for such a purpose. In this paper, recent advances in length-scale-dependent scandium (Sc) microalloying effects in Al–Cu model alloys are reviewed. In coarse-grained Al–Cu alloys, the Sc-aided Cu/Sc/vacancies complexes that act as heterogeneous nuclei and Sc segregation at the θ′-Al₂Cu/matrix interface that reduces interfacial energy contribute significantly to θ′ precipitation. By grain size refinement to the fine/ultrafine-grained scale, the strongly bonded Cu/Sc/vacancies complexes inhibit Cu and vacancy diffusing toward grain boundaries, promoting the desired intragranular θ′ precipitation. At nanocrystalline scale, the applied high strain producing high-density vacancies results in the formation of a large quantity of (Cu, Sc, vacancy)-rich atomic complexes with high thermal stability, outstandingly improving the strength/ductility synergy and preventing the intractable low-temperature precipitation. This review recommends the use of microalloying technology to modify the precipitation behaviors toward better combined mechanical properties and thermal stability in Al alloys.