Reports on analytical methods for determining the geographical origin of agricultural products have been increasing since the 1980s [49]. The development of new techniques is highly desirable for consumers, agricultural farmers, retailers and administrative authorities. Various analytical techniques have been studied based on organic constituents, mineral composition, light- and heavy-element isotope ratios or combinations thereof. The analytical techniques that have been applied for the geographical origin discrimination of food may be subdivided into several groups, mainly including spectroscopic techniques and separation techniques. In general, mass spectrometry (MS) is applied to elucidate the composition of a sample by generating spectral information of the components and can be combined with other techniques. For instance, isotope ratio mass spectrometry (IRMS) is a technique that can distinguish chemically identical compounds based on their isotope content. Continuous flow IRMS (CF-IRMS) is the most common type of IRMS used in food analysis. The CF-IRMS offers online sample preparation and the possibility of interfacing with other preparation techniques, including elemental analysis, gas chromatography and high-performance liquid chromatography. In this chapter, IRMS techniques are described on the scope of food analysis, focussing on important aspects ranging from the recommended terms for the expression of stable isotope ratio measurement results to the principles and techniques of IRMS. Special consideration is given to sample preparation procedures for IRMS analysis and to the work with international stable isotope reference materials. Finally, the most recent developments and applications of this MS technique in food traceability are also addressed.