The Moon forming giant impact marks the end of the main stage of Earth’s accretion and sets the stage for thesubsequent evolution of our planet. The giant impact theory has been the accepted model of lunar origin for 40 years,but the parameters of the impact and the mechanisms that led to the formation of the Moon are still hotly debated.Here we review the principal geochemical observations that constrain the timing and parameters of the impact, themechanisms of lunar formation, and the contemporaneous evolution of Earth. We discuss how chemical and isotopicstudies on lunar, terrestrial and meteorite samples relate to physical models and how they can be used to differentiatebetween lunar origin models. In particular, we argue that the efficiency of mixing during the collision is a key test of giantimpact models. A high degree of intra-impact mixing is required to explain the isotopic similarity between the Earthand Moon but, at the same time, the impact did not homogenize the whole terrestrial mantle, as isotopic signatures ofpre-impact heterogeneity are preserved. We summarize the outlook for the field and highlight the key advances in bothmeasurements and modeling needed to advance our understanding of lunar origin.