Groundwater contributes only a small amount of raw water to public supply in Scotland and Northern Ireland, but it is nevertheless an important resource and is often the only reliable and potable source for rural communities. There is a striking similarity between the geology and structure of lowland Scotland and that of Northern Ireland due to the southwesterly continuation of the Midland Valley graben into Ireland. That similarity is not widely reflected in the hydrogeological conditions encountered in the bedrock aquifers on either side of the North Channel. Groundwater in both Scotland and Northern Ireland is under-utilized except in some isolated aquifer units where demand now warrants formal groundwater management. Knowledge of most of the major groundwater units, in particular of recharge and recharge processes, is not, as yet, sufficient to create operational models on which to base a system of abstraction licensing.