The intensive research on isotopic methods of age determination at a number of laboratories has produced new methods, advances in experimental techniques, and many additional measurements. These developments are reviewed with particular reference to the effect of the new age determinations on the geologic time scale. The age of the planet now appears to be about 4.5 billion years, and the oldest rocks about 3.5 billion. A large number of new measurements on Precambrian rocks provide a basis for interregional correlations. In general the post-Cambrian time scale remains unchanged, but new determinations certify the older ones and reduce the errors...