In December 2006, in response to the preventable threats posed by poor sanitation and hygiene, the Water and Sanitation Program (WSP) launched two large-scale projects, global scaling up handwashing and global scaling up rural sanitation, to improve the health and welfare outcomes for millions of poor people. Local and national governments are implementing these projects with technical support from WSP. The goal of the Global Scaling up Handwashing Project (HWWS) is to reduce the risk of diarrhea and therefore increase household productivity by stimulating and sustaining the behavior of handwashing with soap at critical times for 5.4 million people in Peru, Senegal, Tanzania, and Vietnam. The project aims to test whether this handwashing behavior can be improved among the poor and vulnerable using innovative promotional approaches. In addition, it will undertake a structured learning and dissemination process to develop the evidence, practical knowledge, and tools needed to effectively replicate and scale up future handwashing programs.