  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.
