The hadronization of partons

We review the description of inclusive single unpolarized light hadron production using fragmentation functions in the framework of the factorization theorem. We summarize the factorization of quantities into perturbatively calculable quantities and these universal fragmentation functions, and then discuss some improvements beyond the standard fixed order approach. We discuss the extraction of fragmentation functions for light charged and neutral hadrons using these theoretical tools through global fits to experimental data from reactions at various colliders, in particular from accurate e+e- reactions at LEP, and the subsequent successful predictions of other experimental data, such as data gathered at HERA, the Tevatron and RHIC from these fitted fragmentation functions as allowed by factorization universality. These global fits also impose competitive constraints on alpha_s(M_Z). Emphasis is placed on the need for accurate data from ep and pp(pbar) reactions in which the hadron species is identified in order to constrain the separate fragmentation functions of each quark flavour and hadron species.