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Abstract We summarize results of tephrochronologic studies conducted along the Pacific margin of the conterminous United States during approximately the last 20 years. Tephrochronology, the use of volcanic ash and tuff beds (tephra layers) to correlate and date late Neogene sedimentary and volcanic deposits, has provided numerical age control and relative temporal correlation in many studies of regional geologic mapping, stratigraphy, tectonism, volcanism, and paleoclimate within the Pacific margin of the United States (e.g., Wilcox, 1965; Westgate and others, 1970; Randle and others, 1971; Sarna-Wojcicki, 1971, 1976; Mullineaux, 1974, 1986; Mullineaux and others, 1975; Mehringer and others, 1977, 1984; Crandell and Mullineaux, 1978; Davis, 1978; Porter, 1978, 1981; Izett, 1981; Sarna-Wojcicki and others, 1979, 1984, 1985, 1987). Because individual tephra layers allow precise temporal correlations over great distances, tephrochronologic studies in this region have made it possible to correlate deposits among numerous depositional basins and diverse depositional facies. Furthermore, because deposits of continental and marine environments can be correlated by means of tephra layers, we can compare and equate independently derived provincial and global biostratigraphies that have been developed for the two realms. Correlations of tephra layers also complement or check other dating methods such as biostratigraphy, magneto-stratigraphy, and isotopic ages (Sarna-Wojcicki and others, 1987; also see Sarna-Wojcicki and Davis, this volume). The