For the past 24 years, high-resolution two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) has been the technique of choice for analyzing the protein composition of cells, tissues, and fluids, as well as for studying changes in global patterns of gene expression elicited by a wide array of effectors. The technique, which was originally described independently by O'Farrell and Klose, offers the highest resolution of protein separation and, therefore, plays a central role in “proteomics,” an area of functional genomics that combines a plethora of techniques to resolve (2-D PAGE), identify (peptide sequencing by Edman degradation, mass spectrometry, Western immunoblotting), quantify (scanners, phosphorimagers), and characterize proteins, as well as to store (image analysis and 2-D PAGE databases), communicate, and interface protein data with that emerging from the various genome mapping and sequencing projects. Proteomics, together with genomics, cDNA arrays, phage antibody libraries, and transgenics and knockouts, today provide an impressive array of technologies for studying gene expression in both health and disease.