Isotopic (nuclear) geology constitutes an exact, quantitative branch of the Earth sciences which has expanded rapidly to cover a wide spectrum of applications since the establishment of its basic principles by the early 1950s, in part following seminal researches by H. C. Urey, H. A. Lowenstam, S. Epstein, T. Mayeda and numerous others. This immense progress, accelerating in recent years, led to the application of isotopes in attempting to resolve a variety of geochemical and geological problems in the Earth sciences. Lunar exploration too provided rocks for analysis and their examination stimulated refinements in mass spectrometry later used for terrestrial materials as well. New geochronometric methods were devised and include those based on the radioactive decay of l47Sm to l4:tNd, l76Lu to l7f,Hf, l87Re to ,87Os and 4llK to 4(1Ca, as well as others depending upon the production and distribution of cosmogenic radionuclides such as 2(,A1, l0Be and 36C1.