The Ouse Washes and the surrounding farmland in Cambridgeshire and Norfolk support England's largest concentrations of overwintering Whooper Swans Cygnus cygnus, which migrate to Britain and Ireland from their breeding grounds in Iceland. Using data from the International Swan Census, the Goose and Swan Monitoring Programme, and recent GPS-tracking, this paper examines the subset of the Iceland/UK & Ireland population that winters at the Ouse Washes, by reviewing their migration routes, numbers, breeding success, feeding and roosting ecology, and the pressures that influence their use of the site. The total population size wintering in Britain and Ireland increased from 15,842 in 1995 to over 43,000 in 2020, and the Ouse Washes now regularly supports more than 30% of these birds. This expansion contrasts sharply with the decline of the northeast/northwest European Bewick's Swan Cygnus columbianus population since the mid 1990s. The managed flooding of the Washes and the surrounding farmland provides ideal conditions for wintering birds, though changes in agriculture, hydrology, disease prevalence and climate may reshape future patterns of site use. Continued monitoring and international cooperation are essential to inform the management measures needed to safeguard this key Fenland refuge for Whooper Swans and other migratory waterbirds using the site.