Light scattered from planetary surfaces and atmospheres is polarized while the light of the star is unpolarized. The polarization variations around a planet's orbit provide information that is complementary to that obtainable using spectroscopy. I will describe how polarization could be used in the future to search for liquid water on extrasolar terrestrial planets by detecting the rainbow scattering from cloud droplets and the "glint" from surface oceans. Such observations should be feasible with proposed space missions such as the Terrestrial Planet Finder-Coronograph and provide a means of detecting habitable planets.