Teams from industry, universities and institutes across Europe are contributing to the design and development phase of the European Southern Observatory's project to build the world's biggest optical/infrared telescope. I will outline some of exciting scientific prospects for a fully-adaptive 42m telescope, from studying exoplanets to the furthest galaxies, and then show how some of the technical challenges are being addressed. I will place special emphasis on the work UK teams are doing on instrumentation, detectors and adaptive optics.