The early Mesozoic evolution of the U. S. Cordillera differs greatly from its previous history of mainly miogeoclinal sedimentation with outboard marginal-basin-island-arc mobile zones. The Early Mississippian and Permian-Triassic thrust emplacement of eugeoclinal strata across the miogeocline signaled the initial propagation of subduction-related tectonism onto the sialic edge. Following these events, the sialic edge and the resulting accreted terranes became an active continental margin. The active margin history records not only eastward subduction of oceanic crust beneath North America, but also the formation, migration, and accretion of marginal basin and fringing island-arc systems along the continental margin. At the close of the Jurassic, the fringing arc-marginal basin system collapsed, resulting in a more direct interaction of major Pacific basin plates with hte Cordilleran margin. Such interactions are manifested by Andean and San Andreas types of marginal regimes which characterized the Cretaceous and Cenozoic. In this chapter we will discuss the tectonic evolution of the U. S. Cordilleran margin during the early phases of its active margin history (Triassic through Jurassic).