This document discusses assessment and management of dehydration. It begins by introducing dehydration as a significant cause of morbidity and mortality worldwide, especially in children under 5 from diarrheal illness. It then covers body fluid volumes and compartments, routes of water gain and loss, and types of dehydration including isonatremic, hypernatremic, and hyponatremic. The document also discusses classifying dehydration severity, approaches to management including oral rehydration therapy, and WHO treatment plans for no, some, or severe dehydration. Key aspects of oral rehydration solution mechanism and composition are also summarized.