On 3 October, the Nobel prize for physics was given to three scientists for their discovery that the expansion of the Universe was accelerating, rather than slowing down as expected from current theories of gravitation. There are many ideas for why this might happen, some bundled under the label of Dark Energy, others within modified gravity theories. There are very significant implications for fundamental physics. Euclid is designed to provide the strong observational constraints that are now required to guide physics and cosmology into a new understanding of the nature of the Universe. I will describe how Euclid will address these issues, and provide some insight also into how such a mission is conceived and taken through a stringent selection process.