  Terrestrial planetary systems may exist around nearby stars as the
Earth-sized counterparts to the many giant planets already discovered within
the solar neighborhood. In this chapter we first discuss the numerous
techniques which have been suggested to search for extrasolar terrestrial
planets. We then focus on the expected results from that technique in which an
orbiting telescope or interferometer is used to obtain a visible or infrared
spectrum of a planet, without contamination from the parent star. We show
examples of such spectra for selected cases: the present Earth, the
Neoproterozoic (snowball) Earth, a methane-rich Earth, and the present Mars and
Venus. We conclude by discussing the implications of such spectra for the
detection of life on an extrasolar terrestrial planet.
