The Wadden Sea is an ecotone of far-reaching importance. Its nature is continuously changing, and ecological monitoring is needed to identify and analyse trends, and to inform the general public about accelerated change caused by human impact. Sustained ecological research with a diversified monitoring program is essential to overcome the ‘immature phase’ of short-term and single-site studies in ecological science. Long-term and wide-scale phenomena are of increasing relevance to human welfare. Ecological monitoring includes retrospective and ongoing recordings of physical, chemical and biological parameters on a regional scale. It improves our understanding of natural patterns and processes, and of the distortions caused by man. It may provide early warnings. It is a necessary precondition for translating the principle of anticipatory action into practical policy, and it also documents the effects of actions taken. Monitoring the Wadden Sea will help to protect its great variety of flora and fauna, and will also prevent it from deteriorating into a gray coastal backwater, serving for utility purposes only.