We discuss some general characteristics of modifications of the 4D
Einstein-Hilbert action that become important for low space-time curvatures. In
particular we focus on the chameleon-like behaviour of the massive
gravitational degrees of freedom. Generically there is at least one extra
scalar that is light on cosmic scales, but for certain models it becomes heavy
close to any mass source.Comment: 4 pages, contribution to the proceedings of the Rencontres de
Moriond: Contents and Structures of the Universe, March 18-25, 2006, La
Thuil