Pharmacists play an important role in the assessment and management of diarrhea. By obtaining a focused clinical history from the patient, they can identify patients who need to be referred to their physician or to the emergency room, as well as which patients can be managed at home with oral rehydration therapy (ORT) and sometimes symptomatic therapy. Red flags that prompt referral include severe diarrhea (characterized by one or more of the following: ≥6 stools/day for more than 2 days, fever, severe abdominal cramping, bloody stools, dehydration, or associated with vomiting for more than 4 h), and high-risk patient factors (young children or age > 65, chronic medical conditions, immunosuppression, frailty, or pregnancy). Understanding the categorizations and etiologies of diarrhea is essential for proper assessment and management. Acute diarrhea is defined as diarrhea with <14 days duration. Persistent diarrhea lasts 14–30 days, and chronic diarrhea is defined as >30 days duration of symptoms. Acute diarrhea is most often caused by infection, whereas persistent and chronic diarrhea are more likely to be drug- or disease-related. Diarrhea-associated mortality in high-risk individuals is largely due to dehydration. Preventing and treating dehydration is the cornerstone of management for all patients. Most patients can be managed with ORT, either with oral fluids and salty foods, or with oral rehydration solutions (ORS) that are formulated to maximize rehydration. Moreover, intravenous fluids are needed in patients with severe dehydration.