The purpose of this chapter is to discuss the use of isotope ratio mass spectrometry (IRMS) and isotopic abundances in their application to forensic science and forensic intelligence. Key considerations for developing models and databases for this purpose are discussed, and key thematic examples are used to show the diversity, depth and impact that these types of highly discriminating measurements can have. The chapter ends with a number of case examples, which further support the impact of the use of IRMS in the realm of forensic intelligence.