This chapter discusses how potential habitability can be assessed for approximately terrestrial-sized exoplanets (extrasolar planets). We review the concept of the habitable zone and various definitions of this term. These discussions include limits for "water-worlds," planets with haze- or cloud-rich atmospheres, "dune-like" planets with limited water reservoirs, and planets with "exotic" greenhouse gases composed of H2 and He. Limits on the mass and radius of the planet are also discussed as they relate to the ability of a planetary atmosphere to retain liquid surface water. Finally, this chapter reviews relatively recent research on specific exoplanets and the implications known exoplanets have for planetary climate.