Vertical series of pollen samples from raised bogs at Tregaron, Cardiganshire and Whixall Moss, Shropshire have been analysed and several of the crucial horizons dated by radiocarbon measurements, thus showing some of the vegetational changes which have occurred in the two regions and their approximate duration. These changes are probably the result of human influence during the last few thousand years, and at Tregaron this interpretation is supported by both surface sample analyses and the historical evidence for the changes in the agricultural practices of the area. The different ways and extent to which man modified or determined the vegetation are discussed.