Characteristics of expert panellists.

Despite significant progress in the provision of water, sanitation, and hygiene (WASH) services, 2.3 billion people still lack access to essential services, and many existing services fail to remain functional, reliable, and safe over time. Strengthening WASH systems is critical to ensure long-term service reliability, equity, and resilience. However, empirical evidence on WASH systems strengthening remains limited. This study employed a Delphi process to identify high-priority research questions to guide systems strengthening efforts, future investments, and policymaking. To collate questions on WASH system strengthening, the research team conducted a rapid scoping review of peer-reviewed and grey literature and extracted questions from international conferences. These questions were categorised, reviewed, and refined by the research leads, covering topics such as understanding the functioning of WASH systems, identifying pathways of change, and addressing systemic challenges of resilience, inclusion, sustainability, and governance. A diverse panel of WASH experts participated in a multi-round Delphi process, reaching consensus on key research priorities. The highest research priorities reflected all WASH system building blocks, with each question linked to at least three of the four domains. Five overarching themes emerged: 1) integrating climate resilience into systems strengthening, 2) enhancing gender, equity, and social inclusion in system approaches, 3) strengthening governance, financing, and accountability mechanisms, 4) improving monitoring, evaluation, and measurement of system change, and 5) understanding the political economy of WASH service delivery. This study highlights the growing recognition that WASH systems strengthening requires a broad approach, addressing dynamic linkages between building blocks, context, and actors. It also underscores the importance of improved knowledge sharing to bridge gaps between researchers, implementers, and policymakers and provides a guide to generate evidence that informs sectoral investments and interventions. By addressing key research priorities, it supports evidence-based decision-making to enhance the reliability, inclusion, and sustainability of WASH services.