  Wilsonian effective theories exploit hierarchies of scale to simplify the
description of low-energy behaviour and play as central a role for gravity as
for the rest of physics. They are useful both when hierarchies of scale are
explicit in a gravitating system and more generally for understanding precisely
what controls the size of quantum corrections in gravitational systems. But
effective descriptions are also relevant for open systems (e.g. fluid mechanics
as a long-distance description of statistical systems) for which the
`integrating out' of unobserved low-energy degrees of freedom complicate a
straightforward application of Wilsonian methods. Observations performed only
on one side of an apparent horizon provide examples where open system
descriptions also arise in gravitational physics. This chapter describes some
early adaptations of Open Effective Theories (i.e. techniques for exploiting
hierarchies of scale in open systems) in gravitational settings. Besides
allowing the description of new types of phenomena (such as decoherence) these
techniques also have an additional benefit: they sometimes can be used to resum
perturbative expansions at late times and thereby to obtain controlled
predictions in a regime where perturbative predictions otherwise generically
fail.
