I first came to know of Greg Brennecka in 2007 through my good friend and colleague Lars Borg. Lars mentioned to me that he knew this student intern at the Nuclear Forensics Group at the Lawrence Livermore National Lab who had applied to the Ph.D. program at Arizona State University. Lars obviously thought very highly of Greg, and suggested strongly that we admit him—and I am glad we heeded that bit of advice! Growing up, Greg was fond of spending time outdoors, and he credits his parents for instilling in him the importance of understanding the natural world around him. So it is no surprise that Greg obtained not one, but two Bachelor of Science degrees—the first majoring in Natural Science/Chemistry from Columbia College (1998–2002) and the second in Geological Sciences from the University of Missouri-Columbia (2000–2003). He then went on to get a Master of Science degree from Oregon State University (2003–2006). In 2005, while he was still at OSU, he decided to spend the summer working in the Nuclear Forensics Group at the Lawrence Livermore National Lab. He enjoyed his time at LLNL so much that he continued his collaborative projects there during summer internships for several years, even while he was a graduate student at Arizona State University. Greg was admitted in 2007 as a Ph.D. candidate in the newly established School of Earth and Space Exploration at ASU. The centerpiece of Greg's doctoral work was the careful documentation, and discussion of the implications, of variation in the 238U/235U ratio in meteoritic and terrestrial samples. During his first year, Greg took a course from me on “Meteorites and Cosmochemistry.” For the final project for this course, he independently developed the idea of measuring the uranium isotope compositions of refractory inclusions in chondritic meteorites to search for evidence of the short-lived radionuclide 247Cm (half-life ~15.6 Ma). Now this is exactly the sort of project that Greg loves to dig into: a big question that can be answered with a novel newly developed analytical method. At the time, Dr. Stefan Weyer was visiting Professor Ariel Anbar's laboratory at ASU. Dr. Weyer helped to develop the methodologies for high-precision U isotope analyses using multicollector inductively coupled plasma mass spectrometry and Greg had the opportunity to interact with him. It is to Greg's credit, his innovative thinking, and motivation that he initiated the work on the U isotope systematics in a suite of refractory inclusions from the Allende CV3 chondrite in the Isotope Cosmochemistry and Geochronology Laboratory at ASU. He found that these inclusions recorded a significant variation in the 238U/235U ratio (~3.5‰) that correlated with the Nd/U and Th/U ratios. These results were the first definitive evidence for the presence of live 247Cm in the early solar system (with an initial 247Cm/235U ratio of 1.1–2.4×10−4) and were published in a paper in Science (Brennecka et al. 2010a). The implications of U isotope variation in meteoritic materials were profound, the basis of high-precision Pb-Pb chronology had up to this point relied on the assumption of an invariant 238U/235U ratio (=137.88) in all natural samples. Greg showed that the variation he measured in the U isotope ratios of Allende refractory inclusions required an age adjustment of up to 4–5 Ma for these objects. This in turn has implications for ages obtained using high-resolution extinct chronometers that are anchored to high-precision Pb-Pb ages. Therefore, Greg's work in documenting U isotope variation and investigating its causes in meteoritic and terrestrial materials represents a seminal contribution to cosmochemistry—because of it, we now have a clearer picture of the chronology of the early solar system. In concert with this work, Greg investigated the U isotope compositions of a variety of terrestrial samples (including uranium ores and carbonates) and experimental run products in the Anbar laboratory, and this work led to a better understanding of the potential causes of variation in the 238U/235U ratio in nature (Brennecka et al. 2010b, 2011a, 2011b). In particular, his work on developing U isotopes in carbonates as a proxy for the ocean oxygen content deserves special mention as it has been highly influential in the fields of paleoenvironmental studies and geobiology; he reported the first U isotope profile across the Permian–Triassic boundary in a paper in the Proceedings of the National Academy of Sciences (Brennecka et al. 2011b). He found that ocean anoxia was extensive and developed very quickly before the mass extinction event. His data have since held up, and have opened up the carbonate record for paleoenvironmental applications using U isotopes. From a cosmochemical standpoint, Greg's dissertation research (published in four papers: Brennecka et al. 2010a, 2010b, 2011a, 2011b) highlighted the fact that while chondritic refractory inclusions record a large variation in the 238U/235U ratio from the decay of the short-lived 247Cm, there is the potential that other fractionation processes could also play a role in causing some of the observed U isotope variation in meteoritic materials. Specifically, low-temperature processes associated with redox change (or change in the coordination of U) may cause some variation, although high-temperature (magmatic or hydrothermal) processes did not appear to do so at the level of precision of these studies. All in all, it is rare for an early career scientist to have such a significant impact in two such diverse fields as cosmochemistry and geobiology, but Greg Brennecka has managed to do this. Greg's doctoral research alone would be worthy of the Nier Prize, but of course he did not stop there. Following his Ph.D., and during postdoctoral positions at ASU (2011–2013) and then at LLNL (2013–2014), he led several projects resulting in significant publications, three of which I highlight here. First, he reported the high-precision U isotope compositions for the angrite meteorites, which have been used extensively as age anchors for several high-resolution short-lived chronometers, in a paper in the Proceedings of the National Academy of Sciences (Brennecka and Wadhwa 2012). Second, he extended his study of U isotopes in Allende refractory inclusions by conducting the first systematic investigation of nucleosynthetic isotope anomalies in a range of elements in these objects. He showed that the same inclusions contained excesses of r-process nuclides for some elements, and depletions for other elements. The magnitude and direction of these r-process anomalies were correlated with mass and demonstrated that there must have been at least two different sites of r-process nucleosynthesis. The data additionally required the addition and mixing of specific supernova material to the solar system at a relatively late stage, i.e., after the formation of the first solids. This work was published in the Proceedings of the National Academy of Sciences (Brennecka et al. 2013). Third, he contributed significantly to a comprehensive investigation of the U isotope compositions of a variety of terrestrial and meteoritic materials, which helped to define the average U isotope composition of the Earth and solar system—this study was published last year in Geochimica et Cosmochimica Acta (Goldmann et al. 2015). Since 2014, Greg has been at the Institute for Planetology at the University of Münster, where he has established a laboratory and research group with support from the prestigious Sofja Kovalevskaja Award of the Alexander von Humboldt foundation. This highly competitive award funds research across all fields, including the humanities, and natural and life sciences, and is presented to the most promising early career scientists to allow them to lead a research group at a German university. Given the tremendous legacy of Alfred Nier in the field of isotope studies and mass spectrometry, it is quite fitting that Greg Brennecka is this year's recipient of the Nier Prize—he has not yet met an isotope or a mass spectrometer he did not like (nor is he likely to in the future!). He is being recognized by the Meteoritical Society for his profoundly impactful body of work in the field of cosmochemistry, particularly for the determination of high-precision U isotope compositions of meteoritic materials that have important implications for early solar system conditions and chronology. However, he is also remarkable for the breadth of his scientific output that spans topics in such diverse areas as planetary differentiation processes and chronology, geobiology, and paleoenvironmental studies. Despite all his successes so far, I have no doubt that his best work is yet to come. It is my great pleasure to present Dr. Gregory A. Brennecka as the recipient of the 2016 Nier Prize of the Meteoritical Society.