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 involved. The major problem of correlating the isotopic dates with the sedimentary column remains. The most promising leads in the solution of this problem lie in absolute age determination of intercalated volcanics and carbonaceous shales.

Ages obtained from the various uranium-lead isotope ratios on pegmatite uraninite or samarskite are generally concordant, provided a correction for radon leakage is applied and the proper common lead correction is made. Under such conditions it is highly probable that a true age has been obtained. Where the apparent ages derived from these ratios do not agree, recrystallization with consequent lead loss is indicated.

In the case of pitchblendes similar criteria apply; however, the ease of recrystallization, transportation, and redeposition complicates the interpretation of the apparent age derived from the measurements. Rubidium-strontium ages have been reported which appear too high. Reasons for these anomalies will be suggested. The potassium-argon method has been shown to be feasible during the last year. This may become the most useful of all isotopic chronometers.