Gluon polarization in the nucleon

Gluons can contribute to the nucleon spin and may provide the part missing when only quark spins are taken into account. The gluon helicity distribution can be obtained from QCD fits or from direct measurements in experiments selecting processes sensitive to gluons. Recently several important measurements were completed and others are in progress in fixed target lepton nucleon scattering as well as in collider experiments with polarized protons. Asymmetries are obtained in processes with significant contributions from gluons such as in lepton nucleon scattering the production of D mesons and production of hadrons with high transverse momentum, and in polarized pp scattering the production of jets, $π0$ mesons and direct photons. These results are interpreted in terms of universal gluon helicity distribution, using assumptions and approximations specific for each analysis. The complementarity of the approaches is very important in controling systematic effects and in establishing the present conclusion and interpretation that the data show no signature of very high gluon polarization in the kinematic region of observations.