The 40Ar budget of the Earth offers a powerful and straightforward argument in favour of the existence of a layered structure to the mantle. Whereas K-Ar-isotope systematics have been included as part of various discussions of mantle structure previously a simple and straightforward argument is presented here. Because 40Ar is produced by 40K the amount of K in the Earth may be constrained from 40Ar budgets. The atmospheric 40Ar budget implies that approximately half of all 40Ar produced within the Earth since its formation is retained within the solid Earth. It is argued that this additional 40Ar is located principally in the lower mantle. ¿ American Geophysical Union 1996