  We review recent developments in the theoretical description and
understanding of multi-particle correlation measurements in collisions of small
projectiles (p/d/$^3$He) with heavy nuclei (Au, Pb) as well as proton+proton
collisions. We focus on whether the physical processes responsible for the
observed long range rapidity correlations and their azimuthal structure are the
same in small systems as in heavy ion collisions. In the latter they are
interpreted as generated by the initial spatial geometry being transformed into
momentum correlations by strong final state interactions. However, explicit
calculations show that also initial state momentum correlations are present and
could contribute to observables in small systems. If strong final state
interactions are present in small systems, recent developments show that
results are sensitive to the shape of the proton and its fluctuations.
