I was invited to write this book as part of the Minerals, Rocks and Organic Materials Series of Springer-Verlag by Professor Peter J. Wyllie in 1974. Ophiolites have preoccupied me ever since 1948 as a graduate student and up to the present time as part of my research with the U.S. Geological Survey. During this period ophiolite, an obscure European geological term, has attained an ever-increasing importance, is now used to include all fragments of ancient oceanic lithosphere incorporated into the orogenic zones of modern and ancient continental margins, and is a standard part of the plate tectonic paradigm. The purpose of this book is to provide a starting point for anyone interested in the background and state of knowledge concerning ophiolites (ancient oceanic lithosphere). Because ophiolites represent fragments of old oceanic crust their tectonic setting and age are extremely important in the reconstruction of ancient plate boundaries. Present- day plate tectonic theories involve the generation and disposal of oceanic lithosphere, so that these ancient fragments of oceanic lithosphere can be used directly to reconstruct conditions within the ancient oceans. Since 1970, numerous meetings and conferences directly related to ophiolites have stimulated worldwide interest in the subject. As part of the International Correlation Program, the project "Ophiolites of Continents and Comparable Oceanic Rocks," was initiated by Dr. N. Bogdanov, Geological Institute, Moscow. This project has brought together an international group that has focused on the outstanding problems and is now producing a world map of ophiolite distribution.