The Earth is the only planet, of which we know that life evolved on it. Is there life on other planets, and how can we find it? How does such a planet look like, and which processes are involved? To detect life on other planets (also beyond our solar system), we first concentrate on the planetary surface. To do that, we start from two directions: To select particular planets (out of billions in the milky way) for investigation, we categorize them by their size, distance to their star and its luminosity, on one hand. Which properties matter in terms of habitability? On the other hand, we search for direct and indirect hints (biosignatires) and proofs (biomarkers) for existing life on these planets, like oxygen or ozone in the atmosphere. How does the planet adapt to life? Does life alter the planet in a way that it stays habitable?