PROJECT SUMMARY Background: Effective scalable water, sanitation, and hygiene (WASH) interventions are urgently needed to reduce pediatric diarrheal disease globally. Phone based reminders is an emerging low cost approach shown to lead to improved disease prevention practices. Objective: The purpose of this study is to design, implement, and evaluate a WASH mobile health (mHealth) intervention that can be delivered nationally as a mass phone messaging campaign in Bangladesh to increase handwashing with soap and water treatment practices. Previous studies: Our research group recently developed the Cholera-Hospital-Based-Intervention-for-7-Days (CHoBI7, picture in Bangla), a WASH intervention which promotes handwashing with soap and water treatment to diarrhea patients and their household members in a health facility setting. Our recent randomized controlled trial (RCT) in Bangladesh of CHoBI7 demonstrated this intervention was effective in significantly reducing cholera, and led to sustained handwashing with soap and improved drinking water quality. Building on this work, we are currently conducting a RCT evaluating the impact of delivering mHealth messages to diarrhea patient households as a low cost approach to reinforce the CHoBI7 intervention delivered in a health facility setting. Design and Setting: The Director of Disease Control at the Bangladesh Ministry of Health and Family Welfare is interested in taking our WASH mHealth intervention to scale through incorporating this intervention into the National Operational Plan for Communicable Disease Control. To build evidence for scalable approaches to deliver this intervention our study will be divided into three phases: (1) Formative research and planning; (2) Intervention implementation and evaluation; and (3) Dissemination and policy planning. During the formative research and planning phase will develop an evidence and theory based WASH mHealth intervention through in-depth interviews, focus group discussions, workshops, and a pilot. During the intervention phase we will conduct a two-arm RCT to prospectively follow 300 households and 1200 participants (4 per household) to evaluate the effectiveness of the developed intervention in terms of increasing handwashing with soap, stored drinking water quality, and reducing caregiver hand contamination. The first arm will receive the WASH mHealth intervention developed for the mass phone messaging campaign and the second arm will serve as a control arm. During our dissemination phase, we will work closely with the Director of Disease Control to disseminate the study findings and inform policies for WASH. Significance: This will be the first study to develop a theory and evidence based WASH mHealth intervention that can be delivered nationally as a mass phone messaging campaign.