In 2015, the ecological status of 75-90% of surface waters in NW Europe was reported to be less than good. Diffuse pollution with nitrogen (N) and phosphorus (P) from agriculture, together with point sources of N and P pollution from wastewater plants and industry, are a major cause of this failure to achieve good ecological status. Climate change is likely to exacerbate the pollution risk. Clearly, there is a huge challenge to implement effective mitigation measures to reduce nutrient loadings to air and water to meet desired environmental targets against the background of a changing climate. The synergies and tradeoffs between management actions to mitigate climate change and those to mitigate N and P pollution need to be clarified to develop effective local, regional and national policies. Governance arrangements to implement and monitor the necessary actions need to be better integrated, requiring a consistent and coherent set of environmental indicators. The overall objective of NEW-HARMONICA is to assess and co-develop a harmonised systemic approach to prioritizing an effective suite of N and P pollution mitigation measures and indicators to meet local to regional environmental targets. To achieve this objective, an experienced NEW-HARMONICA consortium (4 partners from 3 countries) with complementary expertise, skills and networks will work on 4 N and P-polluted cross-border river basins in NW Europe. All partners are involved in an established NW Europe Policy-Science Working Group (PSWG) who together with local catchment stakeholders play a central role in NEW-HARMONICA. NEW-HARMONICA’s approach is based on a combination of technical assessments, including quantification of N and P flows and load-reduction targets in the study catchments, assessments of governance arrangements, and policy support for the co-development of harmonised environmental policies, based on a strong evidence base and interactions with the PSWG and local stakeholders.