  Ultra-peripheral collisions (UPC) occur when the interacting nuclei or
protons have an impact parameter larger than the sum of their radii. They are
mediated by virtual photon exchange. The photoproduction of heavy vector mesons
is especially interesting because they couple to the photon. The ALICE
Collaboration has analysed both p-Pb and Pb-Pb UPC at the centre-of-mass energy
of $\sqrt{s_{\rm NN}} = 5.02$TeV which correspond to $\gamma$p and $\gamma$-Pb
interactions, respectively. This poster reports the exclusive photoproduction
of ${\mathrm J}/\psi$ off proton and Pb targets, which shed light on the
occurrence of saturation and nuclear shadowing, respectively. In more detail,
the p-Pb results measure the growth of the cross section for exclusive
production over a wide range in Bjorken-$x$ from ~$10^{-2}$ to ~$10^{-5}$,
while Pb-Pb results demonstrate the presence of nuclear shadowing at high
energies and low scales of the order of the mass of the ${\mathrm J}/\psi$.
