After a decade rich in giant exoplanet detections, observation techniques have now reached the sensitivity to gain information on the physical structure and chemical content of some of the detected planets and also to find planets of less than 10 M?. The detection and characterization of Earth-like planets is approaching rapidly and dedicated space observatories are already in operation (CoRoT) or in the development phase (Kepler, Darwin and TPF-I/C). In this paper, we explore the domain of terrestrial planets, emphasizing habitable worlds. We discuss the possibility of performing a spectral characterization of their properties using the next generation of astronomical instruments.