  Gravity stands out among the fundamental interactions because of its apparent
incompatibility with having a quantum description. Moreover, thermodynamic
aspects of gravitation theory appears as puzzling features of some classical
solutions such as black holes. These and other aspects of gravitational
theories have recently lead to the proposal that gravity might not be a
fundamental interaction but rather an emergent phenomenon, a sort of
hydrodynamic limit of some more fundamental theory. In order to further explore
this possibility we shall here discuss two systems where such emergence of a
gravitational dynamics is observed. We shall consider first the case of a
non-relativistic Bose-Einstein condensate and then a more abstract model based
on scalar fields living on a Riemannian manifold. This will allow us to put in
evidence the general issues related to emergent gravity scenarios with a
particular attention to the role and nature of Lorentz and diffeomorphism
invariance.
