The Carboniferous basins of Britain and Ireland were part of a shallow epicontinental seaway. Widespread shallow- and marginal-marine conditions during the Asbian and Brigantian are recorded by mixed carbonate and siliciclastic deposits across northern Britain. Further south, carbonate successions developed on the platforms and shelves of England, Wales, and southern Ireland. Distinctive changes in the stacking pattern of the cycles defined from these successions at different locations suggest regional transgressions in the Asbian during a period of high global temperatures. In the mid-Brigantian, a marine transgression reduced deposition on the carbonate platforms and flooded the Midland Valley of Scotland, connecting previously separate subbasins. During the latest Mississippian (Namurian), the first extensive Southern Hemisphere glacigenic deposits broadly coincide with a positive shift in the carbon and oxygen stable isotope record and a transition to a humid climate in paleo-tropical latitudes. At this time across Britain and Ireland, siliciclastic fluvial, deltaic, and deep-water deposition dominated, and extensive peat mires developed in Scotland. Large-scale multistory fluvial systems (several tens of kilometers in width and tens of meters in thickness) incised into marine and deltaic deposits during periods of low eustatic sea level, and widespread marine bands are interpreted as eustatic sealevel rises. During the Westphalian, extensive peat mires developed on low-gradient waterlogged depositional plains. The presence of several widespread Westphalian marine bands suggests that significant eustatic rises led to extensive transgressions of this largely nonmarine environment. In the Asbian and Brigantian, eustatic sea-level changes were probably of lower magnitude during a period of higher global temperatures. During the early Namurian, the shift to a much cooler global climate provided a mechanism for generating higher-magnitude and higher-frequency eustatic changes as ice sheets waxed and waned.