The Guidance Note for the Implementation of Source-to-Sea Management in Transboundary Basins has been developed under the programme of work of the Water Convention for 2025–2027 (ECE/MP.WAT/75/Add.1), as part of programme area 3.5: Source-to-Sea – Linking Governance and Management of Transboundary Freshwaters and the Coastal and Marine Environment. It reflects growing recognition of the need to better connect the management of land, freshwater, coastal, and marine environments, especially in basins shared by two or more countries. The guidance note aims to support a better understanding of source-to-sea management and offers practical recommendations for its application. It outlines how this approach can enhance cooperation between upstream and downstream actors, promote integrated management of natural resources, and strengthen ecosystem health in transboundary basins. By emphasizing the linkages between rivers, lakes, aquifers, coasts, and oceans, the guidance note contributes to more holistic, coordinated, and sustainable governance, while at the same time promoting sustainable development, economic growth and facilitating peace.