Thermal ionization mass spectrometry is an established and important tool in the earth sciences. Precise measurements of radiogenic isotope abundances have allowed the development of useful geochronometers and petrogenetic tracers. Further advances in isotope geochemistry will, in part, depend on the capability of analyzing smaller and smaller samples. Techniques highly suitable for small sample/high resolution analysis such as SIMS or AMS will therefore become increasingly more important in the earth sciences. Nevertheless, because of continued instrumentation improvements in conventional mass spectrometry, and the probable high costs and lower accessibility of SIMS/AMS techniques, conventional techniques will remain of major importance in the foreseeable -future.