This report provides a retrospective look at the philanthropic investments of the Conrad N. Hilton Foundation in the water, sanitation, and hygiene (WASH) sector over the last two decades. It summarizes the Foundation's achievements, critical lessons learned, and the evolution of the Foundation's role and strategy based on changes in its own management, feedback from the grantees, and changes in the WASH landscape. Finally,the report provides strategic recommendations for the Foundation's future grantmaking. This document may be viewed as a companion resource to the landscape analysis of the WASH sector completed by FSG Social Impact Advisors in preparation of the Hilton Foundation's new WASH strategy.