  Signatures of collectivity, including azimuthally anisotropic and radial flow
as well as characteristic hadrochemical dependencies, have been observed since
long in (ultra)relativistic nucleus-nucleus collisions. They underpin the
interpretation of these collision systems in terms of QGP formation and
close-to-perfect fluidity. Remarkably, however, essentially all these
signatures of collectivity have been identified within the last decade in
collision systems as small as pp and p-Pb, where collective phenomena had been
assumed to be absent traditionally. Precursor phenomena may have been found
even in ep and $e^+e^-$ collisions. This article provides a complete review of
all data on small system collectivity. It reviews model simulations of these
data where available. However, in the absence of a phenomenologically fully
satisfactory description of collectivity across all system sizes, we focus in
particular on the theoretical basis of all dynamical frameworks of collectivity
invoked in heavy ion collisions, and their expected scaling with system size.
Our discussion clarifies to what extent all dynamical explanations are
challenged by the available data.
