A current understanding of the hydrology of Ogof Draenen, Wales, one of the longest and most complex cave systems in Europe, is presented. Previous tracer tests are reviewed and results of two new tracer tests presented. Numerous dolines occur on the Marros Group (formerly 'Millstone Grit') sandstones and the Pembroke Limestone Group (both of Carboniferous age) that crop out around the edges of the mountains overlying Ogof Draenen, with hydrologically active sinking streams common along the b...