simply been trying to quantize the wrong degrees of freedom [@Hu2; @BVL]. But general relativity has an enormous body of observational support, and models of emergence face formidable challenges in reproducing these successes. The goal of this article is to explain some of these challenges.

This is by no means a comprehensive survey of emergent gravity. The specific models I refer to have been chosen primarily as exemplars for particular issues, and most of the ideas are well known to those working in the field, although they have not been collected in one place. For broader reviews, see [@Sindonirev] or articles in [@Oritibk].

I should also start with a cautionary note. In one sense, *any* quantum theory of general relativity will have “emergent” aspects. All observables (in Dirac’s sense) are necessarily nonlocal [@Torre; @Hartle], so locality must be an emergent property. Moreover, for asymptotically anti-de Sitter spacetimes, and perhaps for asymptotically flat spacetimes as well, the algebra of observables is isomorphic to an algebra of operators at an asymptotic boundary [@Marolf1], so the bulk physics can itself be viewed as emergent from a boundary theory. Such a picture seems contrary to the spirit of emergence, since the boundary degrees of 