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 O2 and the 
simultaneous presence of CH4 traces are suggestive of biology. In this talk we discuss the 
biomarkers at different wavelengths and focus on what makes a habitable planet, using Earth 
as our example and discuss surface, clouds and biosignature evolution over geological time. 
How do we pick our targets? We look at biomarkers on planets around different stars, and set 
the measurements in context with physical characteristics of a planet. The search for signs 
of life implies that we need to gather as much information as possible in order to 
understand how the observed atmosphere works physically and chemically. We investigate how 
to detect habitability in extrasolar planets and how different instrument designs influence 
what you can observe. We live in an exciting time where comparative planetology and the 
search for life on exoplanets is becoming a reality. This talk shows the stepping stones to 
make it a reality.