After reviewing some of the constraints on possible modifications
of gravity coming from laboratory and solar tests, I will present
modified gravity models on large scales. In particular, I will focus
on the DGP and chameleon-f(R) models. In both cases local tests
are evaded thanks to non linear mechanisms: the Vainshtein
effect for DGP and the thin shell phenomenon for chameleons.
Eventually, I will present new results on modifying gravity with a
dilaton in the strong string coupling regime.