  We review the present understanding of the spin structure of protons and
neutrons, the fundamental building blocks of nuclei collectively known as
nucleons. The field of nucleon spin provides a critical window for testing
Quantum Chromodynamics (QCD), the gauge theory of the strong interactions since
it involves fundamental aspects of hadron structure, and it can be probed in
detail in experiments, particularly deep inelastic lepton scattering on
polarized targets. QCD was initially probed in high energy deep inelastic
lepton scattering with unpolarized beams and targets. With time, interest
shifted from testing perturbative QCD to illuminating the nucleon structure
itself. In fact, the spin degrees of freedom of hadrons provide an essential
and detailed verification of both perturbative and nonperturbative QCD
dynamics. Nucleon spin was initially thought of coming mostly from the spin of
its quark constituents, based on intuition from the parton model. However, the
first experiments showed that this expectation was incorrect. It is now clear
that nucleon physics is much more complex, involving quark orbital angular
momenta as well as gluonic and sea quark contributions. Thus, the nucleon spin
structure remains a most active aspect of QCD research, involving important
advances such as the developments of generalized parton distributions (GPD) and
transverse momentum distributions (TMD). Elastic and inelastic lepton-proton
scattering, as well as photoabsorption experiments provide various ways to
investigate non-perturbative QCD. Fundamental sum rules -- such as the Bjorken
sum rule for polarized photoabsorption on polarized nucleons -- are also in the
non-perturbative domain. This realization triggered a vigorous program to link
the low energy effective hadronic description of the strong interactions to
fundamental quarks and gluon degrees of freedom of...
