  Measurements of multi-particle correlations in the collisions of small
systems such as $p+p$, $p/d/^3He+A$ show striking similarity to the
observations in heavy ion collisions. A number of observables measured in the
high multiplicity events of these systems resemble features that are attributed
to collectivity driven by hydrodynamics. However alternative explanations based
on initial state dynamics are able to describe many characteristic features of
these measurements. In this brief review we highlight some of the recent
developments and outstanding issues in this direction.
