During glacial episodes, glaciers are usually initiated in the mountains and spread out, forming extensive kilometer-thick ice sheets inundating the landscape. Most information about their extent has been derived from terrestrial geological and geomorphological evidence. Until recently, there has been a lack of information from the surrounding continental shelves, which, along with a certain reluctance of paleo-glaciologists to allow their ice sheets to get their feet wet, means that it is likely that we have significantly underestimated the extent and volume of some ice sheets. Marine-based ice sheets (i.e., grounded in shallow seas because ice mass outweighs the buoyancy) are widely considered to be less stable than their terrestrial counterparts, and are prone to rapid collapse. This is important, because we know that sudden inputs of fresh water into the North Atlantic can affect its thermohaline circulation. The British-Irish and Fennoscandian ice sheets are fairly well constrained regarding their terrestrial extents, but there has been much discussion concerning the timing and extent of ice cover in the intervening North Sea. We might expect these ice sheets to advance into the sea and to the continental shelf-break, as many others have done, and as some numerical models suggest.