The Dynamics of OB Associations
  The formation and evolution of young star clusters and OB associations is
fundamental to our understanding of the star formation process, the conditions
faced by young binary and planetary systems, and the formation of long-lived
open and globular clusters. Despite this our understanding of the physical
processes that drive this evolution has been limited by the static nature of
most observations. This is all changing thanks to a revolution in kinematic
data quality from large-scale radial velocity surveys and new astrometric
facilities such as Gaia. Here I summarise recent studies of multiple OB
associations from both {\it Gaia} and ground-based astrometric surveys. These
observations show that OB associations have considerable kinematic substructure
and no evidence for the radial expansion pattern predicted by theories such as
residual gas expulsion. This means that, contrary to the standard view of OB
associations as expanded star clusters, these systems could never have been
dense star clusters in the past and were most likely born as extended and
highly substructured groups of stars. This places strong constraints on the
primordial clustering of young stars and the conditions faced by young
planetary systems.
