The terrestrial Mesozoic sediments, floras and faunas of Montana are reviewed and interpreted. Environmental diversity was low, and apparently depended upon the relative position of the interior seaways. With the exception of the areas bordering the seaways where the climates were temperate to subtropical, most of Montana was semi-arid to arid throughout the Mesozoic. The upland coastal plain regions became much drier as the seaways regressed. Faunal and floral diversity was much higher in the lowland regions than in the upland regions, probably because the water ways were much larger in the lowlands, and could support more life.