The measurements of the spin structure functions for the proton and neutron
are reviewed. Recent high-precision data have allowed the Bjorken sum rule
to be verified to within ca. 10% accuracy, providing a rigorous test of qu
antum chromodynamics, the current theory of strong interactions. The data s
how that there is a deficit between the spin carried by the quarks and the
spin of the nucleon, as measured in deep-inelastic scattering. The theories
of the deficit are discussed.