Understanding the spin structure of the proton remains one of the central challenges in Quantum Chromodynamics (QCD). Despite decades of experimental and theoretical efforts, key questions remain open, particularly regarding the contributions from gluons and sea quarks. In this talk, I will discuss recent progress in the precision determination of spin-dependent parton distribution functions, and the role of perturbative QCD in interpreting polarized deep inelastic scattering and proton-proton collisions. I will highlight the impact of new data, global QCD analyses, and the interplay with the future Electron-Ion Collider measurements. Emphasis will be placed on how precision QCD techniques are helping unravel the spin decomposition of the proton, and what remains to be understood in this dynamic area of hadronic physics.Understanding the spin structure of the proton remains one of the central challenges in Quantum Chromodynamics (QCD). Despite decades of experimental and theoretical efforts, key questions remain open, particularly regarding the contributions from gluons and sea quarks. In this talk, I will discuss recent progress in the precision determination of spin-dependent parton distribution functions, and the role of perturbative QCD in interpreting polarized deep inelastic scattering and proton-proton collisions. I will highlight the impact of new data, global QCD analyses, and the interplay with the future Electron-Ion Collider measurements. Emphasis will be placed on how precision QCD techniques are helping unravel the spin decomposition of the proton, and what remains to be understood in this dynamic area of hadronic physics.