This paper is a review of what we know about how the proton’s spin is split among its parts. The authors focus on helicity-dependent, or polarised, parton distribution functions. These functions describe how quarks, antiquarks, and gluons inside a fast-moving proton have their spin aligned or anti‑aligned with the proton’s spin. The review explains how these distributions are defined, how they are extracted from data, and what we still do not know.

A helicity-dependent parton distribution (often called a polarised PDF) is simply the difference between the number of partons whose spin points along the proton’s spin and those whose spin points opposite. Integrals of these distributions give moments such as ∆Σ, the total quark plus antiquark helicity, and ∆G, the gluon helicity. The paper stresses that one important quantity, the flavour‑singlet axial charge a0, is related to these moments but its separation into quark and gluon parts depends on the technical choice called the factorisation scheme. Quantum effects known as the axial anomaly also affect this separation, so how much of a0 is assigned to quarks or to gluons can change with convention.

The authors review the theory and the experiments used to find polarised PDFs. On the theory side they describe the quantum chromodynamics (QCD) framework that underpins the work: factorisation (the split of short‑distance calculable parts and long‑distance PDFs), coefficient functions that encode the short‑distance physics, and DGLAP evolution (the equations that tell how PDFs change with the energy scale). On the experimental side they discuss the main inputs to global fits: inclusive deep‑inelastic scattering (DIS), semi‑inclusive DIS (SIDIS), and polarised proton–proton collisions such as those measured at the Relativistic Heavy Ion Collider (RHIC). The review also covers statistical methods used to combine many different measurements into a single global determination.