Measurement of the spatially-averaged cosmic expansion history cannot -- even in principle -- distinguish dark energy from a modification of general relativity at low energy scales. The only way to test and constrain modified gravity theories is to examine spatial inhomogeneities. This talk will present a cosmological extension of the parameterized post-Newtonian (PPN) formulation of modified gravity theories. The evolution of long-wavelength curvature perturbations of a flat Robertson-Walker spacetime is almost completely determined by the expansion history, regardless of the form of modified gravity. Only one degree of freedom -- corresponding to the PPN $\gamma$ parameter -- is unconstrained. This parameter can be measured on Gpc scales using CMB anisotropy.