Tephrochronology as used here is the study of tephra layers—volcanic ash beds and tuffs—for purposes of correlation and dating of sediments, rocks, and structures. Tephrochronology is used to provide stratigraphic and age control for such studies as regional stratigraphic and tectonic synthesis; determinations of ages, rates, and directions of crustal motions; calibration and evaluation of provincial and regional faunal, magnetostrati-graphic, isotopic, and chemical age dating methods; and correlation of continental and marine faunal and isotopic stages (Sarna-Wojcicki and others, this volume). Tephrochronology is also the study of tephra layers for their own sake: to determine the manner of their eruption and emplacement, their areal distributions; and to infer from their physical, mineralogical, and chemical characteristics the evolution of their parent magmas. Tephrochronology can also be used to determine the eruptive sources of tephra and to determine the frequency of eruptions from specific areas or vents—information necessary for assessments of regional volcanic hazards (Crandall and Mullineaux, 1978; Miller, 1989). Lastly, tephrochronology is important in providing critical age and correlation control in studies of human history—anthropology and archeology (Davis, 1982; Mehringer and Foit, 1990).