We were commissioned by UNICEF and Oxfam to undertake this study to address two overarching questions: 1) What role, if any, should household water treatment and safe storage (HWTS) play in emergency response (in other words, is HWTS a necessary, effective, and suitable intervention for protecting people affected by emergencies compared to other possible interventions)?; and, 2) What are the factors, if any, associated with feasible, and potentially sustained, implementation of HWTS in response to emergencies (e.g., type of emergency, characteristics of setting and affected population, capacity of responders, types of interventions, nature of programmatic support)? We focused particularly on evaluating HWTS interventions implemented in the acute emergency context, within eight weeks of emergency onset. To answer these questions, we investigated HWTS implementations in four acute emergency contexts between August 2009 and March 2010, including: 1) a cholera outbreak in Jajarkot, Nepal; 2) an earthquake in West Sumatra, Indonesia; 3) a flooding event during a cholera epidemic in Turkana, Kenya; and, 4) an earthquake that caused significant internal displacement in Haiti. These emergencies represented a diverse range of emergency situations, geographical settings, affected population size, and HWTS implementation strategy. In each emergency we conducted the following activities: 1) spatial analysis; 2) household surveys; 3) water quality testing; 4) qualitative interviews with water, sanitation, and hygiene (WASH) responders and logistical staff; and, 5) data collection to characterize response costs. We used microbiological improvement of household water quality as our main indicator of HWTS intervention effectiveness. While we are ultimately interested in the health impact of HWTS interventions, assessing health outcomes directly is not possible in the acute emergency context where rapid assessment is vital and diarrheal disease rates are variable. The potential for health benefits can be reasonably inferred if: 1) the intervention reached the target population at risk of waterborne disease due to reliance on unsafe water (coverage); 2) that population used the HWTS intervention (use); and, 3) as a result of such use, household drinking water met WHO guideline values for microbiological water quality. Using an HWTS option to improve household microbiological water quality from above WHO guidelines values before treatment to below WHO guidelines value after treatment is defined in this report as “effective use”.