Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 305. This field trip was conceived as a way to introduce one of the major volcano-tectonic features of the North American continent to visiting scientists from abroad. Its objectives are to allow the participants to observe first hand the geologic relationships relevant to the formation of the Snake River Plain (SRP) and to discuss various interpretations of SRP genesis. The approach to these objectives is to travel the length of the plain from northeast to southwest and to examine in a logical manner, from younger to older volcanic rocks, the relationships important to an understanding of its origin and evolution (Fig. 1). Even though basaltic volcanism is commonly thought of in association with the SRP, this field trip will emphasize the importance of silicic volcanism because of its much greater volume and because of its profound effect on the upper crustal structure of the SRP.