Most hard rocks are or were exposed to deep weathering processes, as in large shields of Africa, India, North and South America, Australia and Europe. It turns out that the hydraulic conductivity of hard rocks is inherited from these weathering processes, within their Stratiform Fissured Layer located immediately below the unconsolidated weathered layer (saprolite) and, to a much lesser extent, within the vertical fissured layer at the periphery of or within pre-existing geological discontinuities (e.g. joints, dykes or veins). This concept unifies the geological and hydrogeological observations and data about hard rocks. Its recognition opens up large perspectives in terms of applied hydrogeology and geology: mapping hydrogeological potential, water well siting, quantitative management and modelling of the groundwater resource, computing the drainage discharge of tunnels and even quarrying in hard rocks.