In connection with large construction works in the Copenhagen and Øresund area a large number of investigations were conducted on the København Limestone Formation. By combining the different investigations made on the København Limestone Formation a renewed analysis of the fracture controlled groundwater flow is given. An analysis of the fracture systems is presented, with regard to the spatial distribution and their genesis. Three main fracture systems are recognized in the København Limestone Formation: An upper brecciated zone, horizontal fractures and vertical fractures. The uppermost part of the limestone has previously always been expected to have a high transmissivity due to glacial brecciation. It is shown that this is not as common as expected. Most of the water in-flow to boreholes occurs as peak in-flow in rather few horizons which are identified as lst-order horizontal fractures generated by unloading. The transmissivity varies stratigraphically within the København Limestone Formation. It is larger in the Upper- and Lower København Limestone Formation than in the Middle København Limestone Formation.