In a famous paper, Sagan analyzed a spectrum of the Earth taken by the Galileo probe, searching for signatures of life. They concluded that the large amount of oxygen and the simultaneous presence of methane traces are suggestive of biology. In this talk we discuss biomarkers and focus on what makes a habitable planet, using Earth as our example. What can we see? How can we detect those faint planets? What happens to a planets around different stars? How do we find them? How do we pick our targets? From the first bacteria to dinosaurs to take-out, what can we find? We live in an exciting time when the detection of life's signatures on worlds orbiting other stars maybe achievable within a generation.