The Mersey Estuary is widely regarded as one of the most polluted estuaries in Europe. After more than 200 years of neglect the tide of pollution is beginning to turn. This report explains the background to pollution of the estuary; examines the physical and chemical processes which influence water quality; highlights reduction in pollution loads to the estuary; and comments on current water quality status. The report looks at improvements which have occurred in recent years and examines future prospects for the estuary.