Wicken Fen in East Anglia, UK is a unique habitat and a wetland of international importance. A comparison of water table measurements made between 1928 and 1931 with contemporary observations indicates that summer minimum water table elevations are between 0.20 m and 0.45 m lower than in the past. The disparity cannot be attributed to differences in rainfall. There are a number of reasons why the Fen may be 'drying', but it was not possible to identify a single dominant cause. A simple water balance model that simulates groundwater levels in different areas of the Fen has been developed. This indicates that although locally important, in most years flows of water from ditches into the Fen comprise less than 10% of total summer inputs and in wet summers there is a net outflow from the Fen into the ditches. Based on an assessment of the water requirements of desired vegetation communities, a more active water management strategy than is presently implemented is proposed. This paper highlights how a better quantitative understanding of ecosystem functioning and the impact of human interventions are a pre-requisite for the development and implementation of sustainable wetland conservation strategies.