Measurements of $\Delta G$

Our present information on the gluon polarisation $\Delta g/g$ is reviewed. The data from fixed-target lepton-nucleon experiments are in context with the recent data from the RHIC polarised pp collider. The main tools to study $\Delta g/g$ in lepton-nucleon scattering are scaling violations of the $g_{1}$ structure functions and longitudinal spin asymmetries in hadron production. Results from high-$p_{T}$ hadron pairs, inclusive hadrons as well as open-charm production are discussed. At RHIC the most precise data presently came from inclusive $\pi^{0}$ 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.