The field of proteomics seeks to define, on a global scale, the levels, activities, regulation, and interaction of proteins in a biological sample. Proteomics is analogous to transcriptomics—the global analysis of mRNA transcripts that arise from the expression of genes in the genome—although the former is considerably more complicated. Whereas the human genome comprises approximately 30,000 genes, there are likely over 100,000 unique proteins in the human proteome due to the multiple ways each gene can be transcribed and translated into proteins by cellular machinery. Because proteins are fundamental components of all living cells, including enzymes, hormones, and antibodies, they are constantly in flux as the body takes in food, metabolizes it, and stores or burns energy. Furthermore, different proteins are produced and expressed at different developmental stages of an organism’s life cycle, from the moment of conception throughout the aging process. Consequently, proteomics is a dynamic, challenging research area. One of the essential tools being used to meet this challenge is mass spectrometry (MS), which is the focal point of the National Center for Toxicogenomics (NCT) Mass Spectrometry Group. There are a large number of proteins in a system at any one time, and they are always changing as an organism eats and metabolizes food, exercises, and sleeps. The proteome—all the proteins expressed in a living system—is therefore unique to the cell or tissue under study. The more narrowly one can define where the proteins are localized within the system (such as in a specific tissue or location in a cell), the easier it is to characterize the specific proteins and quantify their varying levels of expression. Proteomics research at the NCT focuses largely on changes in an organism’s proteome in response to an event such as exposure to an environmental toxicant, in order to advance understanding of how people might respond to chemical exposures in their environment. The Mass Spectrometry Group, led by NIEHS principal investigator Kenneth B. Tomer, employs high-throughput techniques, including MS, to examine hundreds or thousands of protein changes in a large number of samples. Because proteomics techniques produce large amounts of data, sophisticated analysis tools are used to decipher the results and identify key changes in select protein biomarkers that convey valuable information about exposure to harmful chemicals. The MS facility performs analyses for the intramural NIEHS research community, and Tomer also conducts his own collaborative research projects as part of the NCT Proteomics Group.