Palaeoecological data from a range of wetland types were synthesized in order to study the development of wetland ecosystems in southern Ontario. The present-day nature of the wetland system occurring in a topographically defined basin was shown to be controlled in part by basin morphology. Stratigraphic and vegetational (pollen and macrofossil analysis) data were used to compare the sequence of communities occupying basins during the postglacial. In systems where terrestrial peat overlies lake sediment, the broad stages of development were relatively uniform (from a lake to a shallow open water wetland to a herbaceous marsh or fen to a woody plant-dominated community), although the exact nature of each stage varied widely between basins. The accumulation rate of sediment within such basins varied by up to two orders of magnitude during the postglacial. Allogenic and autogenic factors have been acting on these systems throughout the postglacial, and the sensitivity of individual sites to changes in these environmental conditions will also have varied. Even the widespread land clearance associated with European settlement in the area in recent centuries has not affected some wetland systems detectably, while others have either been destroyed or profoundly changed. The patterns of hydroseral development, both timing and direction, were compared with known regional environmental changes.