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During the last century, the numbers of sheep in Britain have increased significantly. Today a large proportion of the total flock is concentrated in the uplands. In terms of wildlife and landscape, Britain's uplands are among the most vulnerable areas. Poor, thin soils, high rainfall, and extremes of climate and slope increase their vulnerability. Damage to land in the uplands is also likely to have adverse effects downstream. In particular fisheries, water resources and flood defences are likely to be put at risk. It is widely recognised that the increased numbers of stock have led to a reduction in biodiversity in the uplands. It is less widely recognised that they may also have caused soil erosion, riverbank erosion, increased run-off reduced aquifer recharge and low river flows. Ultimately, the process of desertification (severe and catastrophic soil erosion) results in hillsides that have been stripped of their vegetation and soils, a process which is accelerating in Britain's uplands. This paper considers the problem by highlighting the processes involved and by presenting case studies from Britain and the USA. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.