This talk will explore the possibility that the dark sector includes new light degrees of freedom (generally scalar fields) that mediate long-range forces on cosmological scales. To ensure consistency with laboratory and solar system tests of gravity, some screening mechanism is necessary to "hide" these degrees of freedom locally. I will focus on two broad classes of screening theories, chameleons and symmetrons, which rely respectively on the scalar field acquiring a large mass or weak coupling in the presence of large ambient matter density. Finally, I will discuss some of the phenomenological/cosmological implications of these theories.