Overview of the science case for a 50-100 m Extremely Large Telescope

We present an overview of the science case for a 50-100 m Extremely Large Telescope. A summary of the potential performance in terms of angular resolution and depth is given. Selected science drivers are then discussed including terrestrial planets in extra-solar systems; stellar populations across the Universe; the first objects and reionization structure of the Universe. Although by no means an exhaustive list, these cases provide examples where an ELT can make a dramatic advance in our understanding of the Universe around us. © Springer 2007