The investigators will enroll a prospective cohort of children under five admitted with diarrhea and dehydration to the Dhaka Hospital of the International Center for Diarrheal Disease Research, Bangladesh (ICDDR,B). Research staff will identify eligible children on arrival and obtain informed consent from their guardian. Children will be weighed on arrival and assessed clinically for the presence or absence of signs of dehydration. A brief ultrasound of the IVC will also be performed. Children will then be rehydrated according to standard hospital protocols, with their weight checked regularly until they achieve a stable, post-hydration weight. Greater than 9% difference between admission weight and stable weight (or post-illness weight) will be considered the gold standard for severe dehydration, against which the investigators will determine the accuracy of our new clinical and ultrasound-based assessment tools.