Spin Dependent Structure Functions of Nucleons and Nuclei

We review recent progress in the understanding of the spin structure of the nucleon. For the free nucleon the issues addressed include the status of the Bjorken and Ellis-Jaffe sum-rules and the role of the axial anomaly. We outline recent work connecting the quark models familiar from hadron spectroscopy to the spin and flavour dependence of the parton distributions. Finally we review the current understanding of nuclear spin structure functions and particularly the extraction of the neutron spin structure function from deuteron data.