Dehydration is responsible for the morbidity and, rarely, mortality secondary to vomiting and diarrhea from acute gastroenteritis. Despite the abundance of literature in the last 20 to 30 years focused on the treatment of dehydration with oral rehydration therapy, few studies have focused on the actual clinical diagnosis of dehydration. Conventional classification systems for assessing dehydration have not been subjected to rigorous prospective validation until recently. This article reviews the recent clinical evidence that validates the use of clinical signs and symptoms in assessing dehydration in children. The utility and limitations of measuring capillary refill time as a sign are also reviewed. In addition, the scant literature examining the utility of laboratory studies in evaluating dehydration is summarized. Oral rehydration therapy for the management of dehydration in children is reviewed. Finally, recent literature examining the effectiveness and advantages of rapid intravenous rehydration in the outpatient setting is discussed.