  The isoscalar $\eta$ and $\eta'$ mesons are special in QCD, being linked both
to chiral symmetry and to non-perturbative glue associated with the axial
anomaly. The properties of these mesons in medium are sensitive to how these
dynamics work in the nuclear environment. In contrast to pionic and kaonic
atoms which are mainly bound by the Coulomb interaction with some corrections
due to the strong force (Coulomb assisted binding), the $\eta$ and $\eta'$ as
neutral mesons can only be bound by the strong interaction. Is this interaction
strong enough? This topic has inspired a vigorous program of experiments,
conducted in close contact with theory, One has to determine the complex
$\eta$, $\eta'$-nucleus potential. Does the real part $V$ provide a
sufficiently deep potential? Is the imaginary part $W$ small enough to allow
for narrow states that can more easily be detected experimentally, i.e. $|W|
\ll |V|$? The $\eta'$ effective mass is observed to be suppressed by $\approx
-40$ MeV at nuclear matter density. Bound state searches are ongoing. This
article gives an overview of the status of knowledge in this field together
with an outlook to future experiments.
