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 quantum chromodynamics, the current theory of strong interactions. The data show 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.