  There is now a substantial body of evidence that a deconfined quark-gluon
plasma is created in ultrarelativistic collisions of heavy nuclei. Some key
observables which are used to gauge the production of the quark-gluon plasma
are the hadronic spectra and multiplicities across many species, azimuthal
anisotropies in the production of various hadrons, jet quenching,
photon/dilepton production, and heavy quarkonium suppression. A key question in
the study of the quark-gluon plasma is what happens to these observables as one
changes the system size. In particular, it is very interesting to study
collisions of `small systems' such as pp, dA, pA, and $e^+ e^-$ since naively
one does not expect to produce a QGP in these cases. One of the surprises from
such studies was the existence of sizable azimuthal anisotropies in the hadron
spectra in the highest multiplicity classes, which led to speculations that one
can generate a QGP in such relatively rare events. This interpretation,
however, is complicated by the fact that there can be multiple sources of
azimuthal anisotropy. I will discuss our current understanding of the different
sources of azimuthal anisotropies and the multiplicity windows in which each
mechanism is expected to dominate. I will also provide a critical discussion of
the reliability of hydrodynamic models applied to small systems. Finally, I
will briefly discuss jet quenching and heavy quarkonium suppression in small
systems and discuss whether or not these observables show any indication of the
production of a small QGP droplet.
