Measurement of the internal spin structure of the nucleon

Recent measurements by the European Muon Collaboration of the spin-dependent structure function gpl(x) of the proton indicate a significant voilation of the Ellis-Jaffe sum rule and imply an entirely unexpected composition of the spin of the proton in the context of the simple quark-parton model. Inclusion of older measurements from SLAC strengthens the EMC result. Based on the combined EMC and SLAC data, the individual quark flavors u+bar u, d+bar d and s+bar s are found to contribute, respectively, +(75±6)% and -(22±6)% of the spin of the proton. All quarks combined account then only (2±19)% of the spin of the proton, while the balance must be due to gluons and/or orbital angular momentum. Several groups have announced plans to measure both the proton and neutron spin-dependent structure functions with the aim to test the fundamental Bjorken sume rule. We discuss several of those possible future experiments