Over the 2011 to 2015 period, the UK government set itself the goal of providing 60 million people with clean water, improved sanitation or hygiene promotion interventions (a type of development assistance known collectively as WASH). In 2015 it reported that it had exceeded this target, reaching 62.9 million people. We conducted an impact review of DFID's WASH portfolio to identify whether its results claims were credible, and to explore whether programmes were doing all they could to maximise impact and value for money. The review concluded that UK aid has made a significant contribution to improving WASH access in low-income countries, and that its claim of reaching 62.9 million people is based on sound evidence. However, the review highlighted sustainability as an area of particular concern, with not enough being done to ensure that improved WASH access was becoming a permanent part of people's lives. It concluded that DFID needs to do more to address long-term problems like water security, maintenance of infrastructure, strengthening local institutions so they can manage services, and changing behaviour. It also found improvements are needed to ensure value for money – with a stronger focus on lifetime investment costs – and to target vulnerable people as well as hard-to-reach communities, in line with the Global Goals commitment to 'leave no one behind'. The review gave DFID a 'Green-Amber' rating, recognising the impressive results, but also underscoring the need to better maximise the impact and sustainability of UK aid in this important sector.