As in the original edition of 1988, this book covers the principles and practice of argon isotopic dating, a technique that has been used to determine the numerical age of the Earth. The greater precision of the 40Ar/39Ar method allows scientists to test smaller geological samples than its precursor, the K-Ar method, which required a comparison of different elements. The new edition will incorporate new developments made in 40Ar/39Ar application over the last decade, made possible by advances in lasers and mass spectrometry.