The Western Interior Seaway accumulated thick sediments which support hydrocarbon sources and reservoirs beginning with the Jurassic Period when the seaway was limited to British Columbia and Saskatchewan Provinces. Seas retreated and advanced throughout the Northern Canadian Provinces up until the Early Cretaceous Period when the seaway began to invade the Western U.S. states. By Late Cretaceous time, the seaway was at its maximum coverage throughout the Great Plains and along the Rocky Mountain Front. The western parts were subjected to deformation while the eastern part remained undisturbed. Deformation along the Rocky Mountain Front promoted fluvial floodplain and swamp deposition along the western shoreline building out deltaic deposits as drainage reached the western shoreline. Coal deposits accumulated. The eastern shoreline consisted mostly of terrestrial deposits reworked as they invaded the seaway. Marine limestone accumulated in the quieter waters while sandstone, shale, and mudstone buried limestone under similar conditions. These sediments promoted hydrocarbon development.The Western Interior Seaway region may be considered to be a largely interconnected hydrocarbon source and reservoir province subdivided into smaller basins covering large portions of states, occasionally crossing state boundaries to extend hydrocarbon source potential within the interior seaway regional province. The history for all basins discussed in this presentation developed under similar geologic settings.Chapter 1 provides an overview of the Western U.S. Mesozoic and Cenozoic geologic history. Chapter 2 describes the stratigraphy and sedimentation associated with the seaway during transgressive-regressive cyclic deposition.Chapter 3 presents coal and petroleum resources of Canada tied to the Western Canadian Sedimentary Basin.Chapter 4 presents petroleum provinces of the Western U.S. developed by the Western Interior Seaway.Chapter 5 summarizes the paleogeography and hydrocarbon development by the seaway.