  Real or virtual photons are excellent probes of nuclear structure, with a
strong sensitivity to gluon distributions. Photonic reactions can be studied
using ultra-peripheral collisions or at an electron-ion collider. Final states
like dijets or open charm production are directly sensitive to the gluon
distributions in nuclei. Exclusive reactions, like exclusive vector meson
production or deeply virtual Compton scattering (DVCS) go further, requiring at
least two gluons. In the Good-Walker paradigm, coherent exclusive
photoproduction is sensitive to the average nuclear configuration (including
gluonic hot spots), and the Fourier transform of the differential cross-section
$d\sigma/dt$ gives the transverse distribution of partonic targets in a
nucleus. The incoherent photoproduction cross-section is sensitive to partonic
fluctuations, including gluonic hot spots. Some reactions, such as dijet
production, involve multiple momentum scales, and thus may be able to probe the
Wigner distribution of nuclear targets. Finally, incoherent photoproduction is
sensitive to partonic fluctuations; an analysis of $J/\psi$ photoproduction on
proton targets found that the data clearly preferred a fluctuating lumpy
proton.
