Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 331. The articles and guide which follows them have been prepared for an eight-day field excursion organized in conjunction with the 28th International Geological Congress in Washington, D.C., in July, 1989. The purpose of the excursion is to review the results of recent research concerning the tectonic, stratigraphic and sedimentological development of the western United States in Late Proterozoic and Cambrian time, and the paleoenvironmental context of the latest Proterozoic and Early Cambrian metazoan radiation event. The location and tectonic setting of stops are shown in the Frontispiece. Our studies build on those of many other geologists, whose contributions are acknowledged in the list of references. In particular, we draw attention to the fundamental contributions over many years of C.A. Nelson, J.H. Stewart and L.A. Wright, and the late M.D. Crittenden, Jr. and S.S. Oriel. Without the basic observations and insights of these individuals, it is unlikely that the present level of understanding would have been achieved. At the same time, we recognize that interpretations and ideas summarized here are subject to revision as research continues.