  Habitability is a measure of an environment's potential to support life, and
a habitable exoplanet supports liquid water on its surface. However, a planet's
success in maintaining liquid water on its surface is the end result of a
complex set of interactions between planetary, stellar, planetary system and
even Galactic characteristics and processes, operating over the planet's
lifetime. In this chapter, we describe how we can now determine which
exoplanets are most likely to be terrestrial, and the research needed to help
define the habitable zone under different assumptions and planetary conditions.
We then move beyond the habitable zone concept to explore a new framework that
looks at far more characteristics and processes, and provide a comprehensive
survey of their impacts on a planet's ability to acquire and maintain
habitability over time. We are now entering an exciting era of terrestiral
exoplanet atmospheric characterization, where initial observations to
characterize planetary composition and constrain atmospheres is already
underway, with more powerful observing capabilities planned for the near and
far future. Understanding the processes that affect the habitability of a
planet will guide us in discovering habitable, and potentially inhabited,
planets.
