Convincing people to adopt preventive health behaviors consistently is difficult, yet many lives could be saved if we understood better how to do so. For example, low-cost point-of-use (POU) technologies such as chlorine and filters can substantially reduce diarrheal disease (Clasen et al. 2006). Nonetheless, they are not widely or consistently used anywhere in the developing world, even when widely available. The authors ran a randomized field study in Kenya in which households received free POU products to test the importance of informational and behavioral constraints on usage. Sharing information about local water quality increases water treatment by 7-10 percentage points (11-24%) above that achieved by providing free products. Persuasive social marketing messages that harness findings from behavioral economics increase water treatment by an additional 9-11 percentage points. These results suggest promising avenues for incremental improvements in encouraging water treatment (and possibly other preventive health) behaviors. However, repeated exposures may be necessary to sustain behavior change.