The authors first trace the history of the application of mass spectrometers in analytical problems, beginning with the work of J.J. Thomson in 1913. This is followed by a description of the theory of mass spectrometry, which is a technique that produces a beam of gaseous ions from a sample (solid, liquid, or gas), sorts out the resulting mixture of ions according to their mass-to-charge ratios, and provides output signals that are directly proportional to the relative abundance of each ionic species present. A discussion of the advantages of mass spectrometry as a forensic tool focuses on its extreme sensitivity, its response to metals and nonmetals, its uniform sensitivity, and minimal matrix effect. Equipment and experimental conditions are then profiled, followed by discussions of the interpretation of the mass spectrum, precision and reproducibility, and forensic sample problems. A review of the materials studied focuses on considerations in the analysis of hair, nails, and human blood serum. 7 tables and 35 references