The fundamental principles of tephrochronology were de- veloped in Iceland through the classic research of Professor Sigurdur Thorarinsson in the 1940's and 1950's. By his studies on the volcanic ash layers (tephra) produced by the historic explosive eruptions of the volcano Hekla, he established that individual tephra layers could be correlated over large areas of Iceland. As the deposition of such layers was essentially instantaneous on a geological time-scale, the tephra layers provided a distinctive and widespread isochronous stratigraphic marker. Since Thorarinsson's pioneering work, the study of tephra has become a powerful and increasingly important research tool in many branches of the geological sciences and related disciplines. In response to the recent rapid growth and diversifica- tion of tephra studies we decided to coordinate a NATO Advanced Studies Institute under the auspices of the INQUA Commission on Tephra. The ASI, entitled "Tephra Studies as a Tool in Quaternary Research," was held from June 18th to June 29th, 1980, at Laugarvatn in Iceland. This book represents the proceedings of that Institute and is the first book dedicated to tephra studies, its many uses and applications. The subject matter combines disciplines as diverse as Quaternary strati- graphy, isotope geochronology, petrology, deep-sea geology, volcanology, volcanic hazards mapping, archaeology and ecology in one volume, and illustrates both present uses and future potential of tephra research.