There has been a resurgence of interest in the relativistic collisions of small
systems after the surprising discovery of a long-range rapidity correlation in
high-multiplicity proton-proton collisions at the LHC.  The same signal was
also seen in p+Pb collisions and d+Au collisions at RHIC which will follow up
with p+Au and $^3$He-Au in the near future.  Multi-particle correlation
measurements suggest the presence of collective phenomena in these systems.
One possible explanation is hydrodynamic flow.  Another is intrinsic
correlations generated from saturated gluonic matter within the proton.  This
talk will provide an overview of the experimental findings and interpretations.