Since the days of Kant and Laplace it has been theorised that our Solar System formed from a swirling disk of gas and dust. We can now find evidence of such disks and planets around other stars. Any theory of planet formation has to explain how such a disk gave rise to our own Solar System with outer gas giant and icy planets, as well as inner rocky terrestrial or Earth-like planets. Nowadays it is recognised that terrestrial planets had different accretion histories. Whereas Mars formed within about a million years, Earth’s Moon formed between thirty and two hundred million years after the Sun. Earth’s last major accretion event is thought to have been the Moon-forming Giant Impact between the proto-Earth and another smaller planet, sometimes called “Theia”. Despite considerable progress in our understanding, advances in isotope-chronometry and computer simulations of planet formation are giving diverging views of lunar formation. The net result is that currently we have no satisfactory explanation for the origin of the Moon.