  OB associations are low-density groups of young stars that are dispersing
from their birth environment into the Galactic field. They are important for
understanding the star formation process, early stellar evolution, the
properties and distribution of young stars and the processes by which young
stellar groups disperse. Recent observations, particularly from Gaia, have
shown that associations are highly complex, with a high degree of spatial,
kinematic and temporal substructure. The kinematics of associations have shown
them to be globally unbound and expanding, with the majority of recent studies
revealing evidence for clear expansion patterns in the association subgroups,
suggesting the subgroups were more compact in the past. This expansion is often
non-isotropic, arguing against a simple explosive expansion, as predicted by
some models of residual gas expulsion. The star formation histories of
associations are often complex, exhibit moderate age spreads and temporal
substructure, but so far have failed to reveal simple patterns of star
formation propagation (e.g., triggering). These results have challenged the
historical paradigm of the origin of associations as the expanded remnants of
dense star clusters and suggests instead that they originate as highly
substructured systems without a linear star formation history, but with
multiple clumps of stars that have since expanded and begun to overlap,
producing the complex systems we observe today. This has wide-ranging
consequences for the early formation environments of most stars and planetary
systems, including our own Solar System.
