The article reviews the proton spin problem in Quantum Chromodynamics (QCD), focusing on the transition between current quarks and constituent quarks. It begins by introducing the spin decomposition of the proton, which includes contributions from quark and gluon spins, as well as orbital angular momentum. The author discusses the deep inelastic scattering (DIS) framework, where the nucleon's internal structure is described by parton distributions, and the role of the axial anomaly in QCD. The axial anomaly induces gluonic contributions to the flavor-singlet axial charge \( g_A^{(0)} \), which is measured in polarized DIS experiments. The small value of \( g_A^{(0)} \) extracted from these experiments is compared with semi-classical predictions, highlighting the need for a better understanding of the spin structure of the nucleon. The article also explores the relationship between chiral symmetry and the spin structure of the nucleon, and the role of gluon topology in the axial anomaly. Finally, it discusses the experimental program aimed at measuring the contributions to \( g_A^{(0)} \) from quark and gluon partons, as well as the topological term, to better understand the internal spin structure of the nucleon.The article reviews the proton spin problem in Quantum Chromodynamics (QCD), focusing on the transition between current quarks and constituent quarks. It begins by introducing the spin decomposition of the proton, which includes contributions from quark and gluon spins, as well as orbital angular momentum. The author discusses the deep inelastic scattering (DIS) framework, where the nucleon's internal structure is described by parton distributions, and the role of the axial anomaly in QCD. The axial anomaly induces gluonic contributions to the flavor-singlet axial charge \( g_A^{(0)} \), which is measured in polarized DIS experiments. The small value of \( g_A^{(0)} \) extracted from these experiments is compared with semi-classical predictions, highlighting the need for a better understanding of the spin structure of the nucleon. The article also explores the relationship between chiral symmetry and the spin structure of the nucleon, and the role of gluon topology in the axial anomaly. Finally, it discusses the experimental program aimed at measuring the contributions to \( g_A^{(0)} \) from quark and gluon partons, as well as the topological term, to better understand the internal spin structure of the nucleon.