A major aim of lunar science has been to understand the early evolution to the lunar crust in the period prior to the extrusion of the mare basalts. There are two aspects to this early period of evolution about which age determinations provide information. On the one hand is the magmatic activity which led to the chemical differentiation of the outer regions of the Moon, while on the other is the bombardment of the Moon by large objects in the period immediately following its formation. The two aspects are probably not unrelated in that the bombardment may represent the final stages of the accretion of the Moon, and the heat source responsible for the initial differentiation was possibly the gravitational energy released during the major accretion phase. 40Ar-39Ar ages have been largely reset by the final stages of the bombardment and therefore most of the information obtained from argon measurements pertains to the chronology of the bombardment. Information on the magmatic activity is obtained from Rb-Sr, U, Th-Pb and Sm-Nd studies.