Diarrhea is one of many symptoms that may complicate the hospitalization of a critically ill patient. Diarrhea is caused by a variety of etiologies; identifying the etiology aids in the appropriate selection of interventions. Care of the patient with diarrhea should be guided by the evidence and best practices available in the literature. This article defines and describes diarrhea and its pathophysiology. An evidence-based plan of care for the assessment, planning, intervention, and evaluation of the patient with diarrhea is presented, using levels of recommendation based on the strength of the available evidence. A case study is presented to illustrate application to clinical practice. Commentary about the case is provided to review salient points of care.