<p>One of the most significant advances in inductively coupled plasma mass spectrometry (ICP-MS) is the recent development of tandem ICP-MS (ICP-MS/MS) instruments using reaction-cell technology. In ICP-MS/MS, collision/reaction gases eliminate spectral interferences by promoting chemical reactions or collisional dissociation with interfering ions. The choice of gas depends on the analyte and the type of interference. This development expands the capability to measure isotopes, free from isobaric interferences, across the periodic table, practically from Li to U, with detection limits reaching pg/ml or sub-pg/ml. There has been a surge in the application of this analytical technique in a large range of science and technology research fields over the last decade. Using collision/reaction gases like He, O₂, H₂, SF₆, N₂O, NH₃, and benzene vapour, even the most difficult interferences can be eliminated either by measuring reaction products in a mass-shift approach, negating interferences via a charge transfer, or by reacting interferences and measuring analytes in on-mass mode. For geological applications, a large range of trace element isotopes, including large ion lithophile elements (LILE), high-field strength elements (HFSE), rare-earth elements (REE), and platinum group elements (PGE) are routinely determined in different types of geological materials with high accuracy and precision. This new analytical capability also opens opportunities for in situ isotope studies, geochronology, and accessing decay systems such as Rb-Sr, Lu-Hf, and Re-Os, which could previously only be accessed by laborious chemical dissolution and separation methods. The novel ICP-MS/MS approach is fast and cost-effective when compared to more established techniques such as high-resolution ICP-MS (HR-ICP-MS), thermal ionisation mass spectrometry (TIMS), sensitive high-resolution ion microprobe (SHRIMP) or multi-collector-ICP-MS (MC-ICP-MS), both in terms of equipment and running costs. This work presents an assessment of the analytical performance of ICP-MS/MS in geological (including geochemical, geochronological, and mineral exploration), environmental, and several other areas of science and technology, benchmarked against the performance of other plasma-based instrumentation such as HR-ICP-MS and MC-ICP-MS. This article also includes several other examples and case studies to showcase the far-reaching potential of ICP-MS/MS.</p>