Towards a Global QCD Analysis of Helicity-Dependent Parton Densities

A solid understanding of the theoretical framework and its phenomenological inputs has been crucial for the success of perturbative QCD in the past and continues to be vital for ongoing and future nuclear and high-energy research programs. Taking single-inclusive hadron production as an example, I will briefly discuss recent progress in extracting fragmentation functions from data. Next, I will review the current status of spin-dependent parton densities and outline the steps towards a global QCD analysis.