ar X iv : h ep - e x / 06 12 05 5 v 1 2 2 D ec 2 00 6 Measurements of ∆

Our present information on the gluon polarisation ∆g/g is reviewed. The data from fixedtarget lepton–nucleon experiments are in context with the recent data from the RHIC polarised pp collider. The main tools to study ∆g/g in lepton–nucleon scattering are scaling violations of the g1 structure functions and longitudinal spin asymmetries in hadron production. Results from high-pT hadron pairs, inclusive hadrons as well as open-charm production are discussed. At RHIC the most precise data presently came from inclusive π0 and jet production. All data indicate that the gluon polarisation is small compared to earlier expectations, but still can make a major contribution to the nucleon spin.