This chapter contains sections titled: Capability of Inorganic Mass Spectrometry in Isotope Ratio Measurements Limits for Precision and Accuracy of Isotope Ratio Measurements and How to Solve the Problems Isotope Ratio Measurements by Gas Source Mass Spectrometry Isotope Ratio Measurements by Quadrupole based ICP-MS Isotope Ratio Measurements by Laser Ablation ICP-MS Multiple Ion Collector Mass Spectrometry for High Precision Isotope Ratio Measurements Applications of Isotope Dilution Mass Spectrometry Isotope Ratio Measurement of Long-Lived Radionuclides Applications of Isotope Ratio Measurements in Geochemistry and Geochronology References