The Pleistocene Boundary and the Beginning of the Quaternary presents the results of a forty-year international program to establish a precise definition for the most important boundary in geologic time - the beginning of the modern age of glacial climates, in which our landscape, our biological environment, and our own speceis were shaped together. This book, the final report of International Geological Correlation Program, Project 41, describes the selection of a fixed reference point in the sequence of deep-water marine strata exposed at Vrica, Italy. This point, dated to 1.81 million years before the present, coincides with the onset of the first of four major glacial phases and with the emergence of Homo erectus in Africa. The precise identification of the Pleistocene boundary has already led to its redefinition in major areas, such as eastern Europe, China, and India, where the boundary previously was recognized at various levels, according to local interpretations of evidence for "the beginning of glacial climates."