[en] Over the past few years there has been intense experimental activity focused on measuring the spin-dependent structure functions of the proton and neutron, triggered by results reported in 1987 by EMC that implied only a relatively small fraction of the proton spin is due to the spins of the constituent quarks. Recent experiments have been performed with longitudinally polarized beams of electrons (at SLAC) or muons (at CERN) incident on longitudinally or transversely polarized nucleon targets, and have yielded substantial improvements in the experimental errors on the measurements. The latest results from these experiments will be reviewed and compared with predictions of the Bjorken sum rule, the Ellis-Jaffe sum rules, and expectations of the naive quark model. Prospects for the future will be discussed