Acute diarrhea is still a prevalent disease with high mortality rates, mostly among young children. Its incidence is related to poor sanitation and low nutritional status, conditions that are found and most parts of Brazil. The clinical classifications, giving the importance of medical history and exams, are reviewed. The main pathogens are described. Treatments focus on hydric replacement, loss prevention, and specific therapy against the pathogens. This review intends to reinforce fundamental data on acute diarrhea, an important and frequent entity in daily practice.