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.