A wide range of cosmological observations imply that the expansion of the Universe is accelerating -- if they are interpreted within a homogeneous and isotropic FLRW model. On considering inhomogeneous models, however, we find that "acceleration" becomes a rather ambiguous concept. In this talk, I describe how the question of whether spacetime is accelerating depends on what measure of acceleration is being used, and explain how this affects attempts to solve the dark energy problem.