Nucleon polarised parton distribution functions

An introduction to the present status of spin-dependent parton distribution functions is given. Both, results from lepton scattering experiments and from polarised proton-proton collisions with longitudinal polarisation are included. Particular emphasis is put on recent results for the gluon polarisation, which is a key element in the nucleon spin structure. Very large gluon polarisation scenarios, put forward to explain the small quark spin contribution to the nucleon spin, are now strongly disfavoured.