  The role of the $U(1)_A$ anomaly in QCD phenomenology is reviewed, focusing
on the relation between quark dynamics and gluon topology. Topics covered
include a generalisation of the Witten-Veneziano formula for the mass of the
$\eta'$, the determination of pseudoscalar meson decay constants, radiative
pseudoscalar decays and the $U(1)_A$ Goldberger-Treiman relation. Sum rules are
derived for the proton and photon structure functions $g_1^p$ and $g_1^\c$
measured in polarised deep-inelastic scattering. The first moment sum rule for
$g_1^p$ (the `proton spin' problem) is confronted with new data from COMPASS
and HERMES on the deuteron structure function and shown to be quantitatively
explained in terms of topological charge screening. Proposals for experiments
on semi-inclusive DIS and polarised two-photon physics at future $ep$ and
high-luminosity $e^+ e^-$ colliders are discussed.
