Summary: Acute diarrhea is one of the most common childhood diseases. It is usually caused by acute gastroenteritis (AGE). Rotavirus is the most frequent agent, but norovirus has become a leading cause of AGE in countries with high rotavirus vaccine coverage. Acute diarrhea of mild or moderate severity in children without underlying comorbidities or dysentery (as indicated by bloody stools) does not require routine microbiological treatment or other laboratory work-up. In the treatment of acute diarrhea, fast return to normal hydration and prevention of further dehydration is crucial. Oral rehydration (with oral rehydration solution) is the key treatment. When oral treatment is not feasible, enteral rehydration by nasogastric route should be considered before intravenous therapy. Regular oral feeding with no dietary changes is recommended. Breastfeeding should be continued throughout the disease. The use of antiinfective drugs may be considered in patients with selected clinical conditions. Treatment with antiemetics (ondansetron) and antidiarrheal medication, including probiotics with documented efficacy (e.g., Lactobacillus rhamnosus GG, Saccharomyces boulardii, Lactobacillus reuteri DSM 17938), racecadotril (antisecretory effect), and diosmectite (adsorbent effect), may be considered to reduce the duration and severity of symptoms. Improving hygiene conditions and the use of rotavirus vaccines are the most important in the prevention of AGE. Acute diarrhea may be also caused by the newly discovered SARS-CoV-2 virus, the presence of which is found in about 10% of children with COVID-19, although the significance of these findings remains unclear (based on the data at the time of writing this article).