Since the end of World War II isotope geology has grown into a diversi­ fied and complex discipline in the earth sciences. It has progressed by the efforts of a relatively small number of specialists, many of whom are physi­ cists, chemists, or mathematicians who were attracted to the earth sciences by the opportunity to measure and to interpret the isotopic compositions of certain chemical elements in geological materials. The phenomenal growth of isotope geology during the last 25 years is an impressive indi­ cation of the success of their efforts. We have now entered into a new phase of development of isotope geology which emphasizes the application of the new tools to the solution of specific problems in the earth and planetary sciences. This requires the active participation of a new breed of geologists who understand the nature and complexity of geological problems and can work toward their solution by a thoughtful application of the principles of isotope geology. It is there­ fore necessary to explain these principles to earth scientists at large to enable them to make use of the new information which isotope geology can offer them.