Despite some progress, universal access to safe water, sanitation and hygiene (WASH) by 2030-a remit of Sustainable Development Goal 6-remains a distant prospect in many countries. Policy-makers and implementers of the WASH sector are challenged to track a new path. This research aimed to identify core orienting themes of the sector, as legacies of past processes, which can provide insights for its future. We reviewed global policy, science and programmatic documents and carried out 19 expert interviews to track the evolution of the global WASH sector over seven decades. We situated this evolution in relation to wider trends in global health and development over the same time period.With transnational flows of concern, expertise and resources from high-income to lower-income countries, the WASH sector evolved over decades of international institutionalisation of health and development with (1) a focus on technologies (technicalisation), (2) a search for generalised solutions (universalisation), (3) attempts to make recipients responsible for environmental health (responsibilisation) and (4) the shaping of programmes around quantifiable outcomes (metricisation). The emergent commitment of the WASH sector to these core themes reflects a pragmatic response in health and development to depoliticise poverty and social inequalities in order to enable action. This leads to questions about what potential solutions have been obscured, a recognition which might be understood as 'uncomfortable knowledge'-the knowns that have had to be unknown, which resonate with concerns about deep inequalities, shrinking budgets and the gap between what could and has been achieved.