  The goal of finding and characterizing habitable planets in other solar
systems represents one of humanity's greatest scientific challenges. NASA and
ESA have initiated studies of missions that could accomplish this goal within
the next ten years. What precursor knowledge do we need before we can initiate
such a mission? How large should the first steps be in a program whose ultimate
aim is to detect life on other planets? This talk describes different concepts
for NASA's Terrestrial Planet Finder and discusses potential precursors in a
program that balances scientific return, technological advance, and
programmatic risk.
