  The detection and characterization of Earth-like planet is approaching
rapidly thanks to radial velocity surveys (HARPS), transit searches (Corot,
Kepler) and space observatories dedicated to their characterization are already
in development phase (James Webb Space Telescope), large ground based
telescopes (ELT, TNT, GMT), and dedicated space-based missions like Darwin,
Terrestrial Planet Finder, New World Observer). In this paper we discuss how we
can read a planets spectrum to assess its habitability and search for the
signatures of a biosphere. Identifying signs of life implies understanding how
the observed atmosphere physically and chemically works and thus to gather
information on the planet in addition to the observing its spectral
fingerprint.
