With 6.6 billion episodes and 1.5 million deaths annually, diarrheal diseases remain one of the leading causes of death and disability in both children and adults worldwide. While the vast majority of diarrheal episodes follow a relatively benign course, approximately 35% of cases in younger children and 5% of cases in older children and adults lead to moderate or severe disease requiring advanced medical management. Appropriate rehydration with either oral or intravenous fluids remains the most important step for treating acute diarrhea. Accurate assessment of dehydration status has been shown to reduce the morbidity and mortality that results from inappropriate rehydration of patients and can also improve the cost-effectiveness of diarrhea management, especially in low resource settings. Yet no validated tools exist for estimating dehydration severity in patients with acute diarrhea across the lifespan. Our prior research derived and internally validated new clinical diagnostic models for dehydration severity both in children under five years of age (DHAKA models) and patients over five years of age (NIRUDAK models) and demonstrated that they were more accurate and reliable than the World Health Organization guidelines, currently the standard of care for managing acute diarrhea in most settings worldwide. We then incorporated our new clinical diagnostic models into a simple mobile health application (mHealth) clinical decision support tool (CDST) that can be used by frontline providers to guide management for patients with acute diarrhea. Building upon the success of our prior work, the proposed R01 renewal award research will externally validate the accuracy, reliability, and clinical utility of our novel mHealth CDST in a high diarrheal burden setting (Tanzania). In addition, we will use an implementation science-based approach to better understand the barriers and facilitators to uptake and usage of the mHealth CDST by a variety of different types of healthcare providers in Tanzania. Once validated, our new mHealth CDST has the potential to reduce both the morbidity and mortality that occurs as a result of missed diagnoses of dehydration, as well as the adverse events and inappropriate utilization of limited healthcare resources that can result from inaccurate diagnoses of dehydration. This can potentially improve treatment for the hundreds of millions of patients presenting to healthcare facilities worldwide with acute diarrhea each year.