Abstract The Cordilleran orogen lies on the western part of the North American continent and rims the northeastern Pacific Ocean Basin. That part of the orogen covered in this volume extends between the Mexican and Canadian borders, with some consideration of the geology on both sides of the border, and from the offshore continental borderlands of the Pacific eastward as far as the Black Hills of South Dakota and the mountains of west Texas. It ranges from 800 to 1, 600 km wide and is physiographically complex, consisting of high mountains, intervening lowlands, and plateaus that rise from a more gentle continental interior (Fig. 1). The physiography of the Cordillera largely reflects the younger underlying structure, and the present orogenic belt has limits coincident with its physiography.