Water, sanitation and hygiene (WASH) system strengthening in low- and middle-income countries (LMICs): a systematic review on the state of the evidence

System strengthening has become a central objective of water, sanitation and hygiene (WASH) policy and programming, yet the evidence base for its effectiveness remains poorly consolidated. To address this gap, we conducted a systematic review of studies that assessed outcomes arising from WASH system strengthening (WASH-SS) interventions, with a study follow-up period of at least six months. Our search identified a limited body of evidence: only 17 studies met the inclusion criteria, and these displayed substantial variation in focus, methodological approaches and contextual settings. We mapped reported outcomes along a logic model and found that the most reported outcomes clustered around process-level changes, systems outputs and intermediate outcomes, whereas distal health and socioeconomic impacts were rarely reported. This distribution partly reflects the complex nature of WASH system strengthening and the difficulty of evaluating distal outcomes, a challenge also found in the health systems strengthening literature. Notably, the evidence suggest that domestically driven reforms are more closely associated with sustained, long‑term outcomes and successful financial leveraging, compared to initiatives driven by international organisations. In doing so, the review offers a replicable logic model for classifying WASH-SS outcomes and a preliminary signal that domestically embedded reforms are more closely associated with sustained outcomes and financing leverage. Overall, the review highlights the need for more robust, longitudinal studies capable of tracing the pathways through which system strengthening contributes to lasting, equitable WASH outcomes.