The aim of the quality status report is to inform about the actual status of the harbour seal, grey seal and harbour porpoise within the Wadden Sea Area of Denmark, Germany and the Netherlands. As top predators, they are sentinels for ecosystem health. A variety of marine mammal experts of all three countries have evaluated and assessed the research data collected during the current reporting period to provide an overview of the status of the three species within the Wadden Sea region. For seals, research methods include surveys following established monitoring methods to provide information on abundance and distribution. For porpoises, information was collated from aerial survey work, passive acoustic monitoring, tagging, and stranding networks. Population biology and health parameters were collected during necropsies of stranded marine mammals. Monitoring methods and coverage differed between regions and countries. Area-wide and continuous monitoring are essential for being able to observe changes and possible negative developments and allow for timely measures for the conservation of the three marine mammal species within the Wadden Sea Area. Marine mammals in the North Sea face a wide range of threats: underwater noise, fishing and pollution (contaminants and litter) which can affect their health and influence their distribution. Despite the current anthropogenic impacts, harbour seal counts during moult revealed the highest numbers since the first assessment in 2000. The same is true for the grey seal population: here, a constant rise since their return to the Wadden Sea in the mid-20th century was observed. For harbour porpoises, a southward shift was detected during large scale SCANS surveys between 1994 and 2016. German aerial surveys revealed a decreasing trend in porpoise density in the Natura2000 site “Sylt Outer Reef” west of Sylt as well as a decline in the entire German North Sea. Telemetry data on six harbour porpoises indicated residency in the Wadden Sea waters. Based on the current status of the harbour seal, grey seal and harbour porpoise, recommendations for the protection of these marine mammal species were formulated.