This evidence synthesis identifies, synthesizes and evaluates existing evidence of the impacts of water, sanitation, and hygiene (WASH) interventions in disease outbreaks in 51 humanitarian contexts in 19 low and middle-income countries. The research team developed theories of change for the WASH interventions under consideration, documenting the theoretical route from intervention activities to outputs, outcomes, and impacts. WASH interventions consistently reduce both the risk of disease and the risk of transmission in outbreak contexts. Programme design and beneficiary preferences are important factors in ensuring WASH interventions reach their potential. Overall, the amount and quality of evidence of the health impacts of WASH interventions in outbreaks is found to be lacking and low. The weak evidence base is attributed to (1) the prioritization of rapid response activities over research in emergency contexts and (2) the difficult of conducting research in the rapidly changing and unstable settings where disease outbreaks often occur.