  In this proceeding, I review what we have learned, and what we will be able
to learn, about low-$x$ physics from proton-nucleus ($pA$) collisions at RHIC
and the LHC, and how this complements future Electron-Ion Collider (EIC)
measurements. EIC not only will provide us essential and complementary
information about the gluon saturation to the knowledge that we have learned
from $pA$ collisions at RHIC and the LHC, it will also allow us to visualize
the internal structure of protons and nuclei in a multi-dimensional fashion
with unprecedented precision.
