This paper is concerned with the dangers of pollution of groundwater which may arise in aquifers composed of massive well-jointed limestones. In the opinion of the author, these dangers arise proncipally from the comparatively unusual hydrology of such aquifers. Therefore, in the discussion which follows, attention will be paid mainly to hydrological facts and arguments rather than to the effects of particular pollutants. The bias of the argument will be towards the reasons why limestone aquifers may be particularly prone to pollution, rather than to instancing examples of such pollution occurring at the present time.