Diarrhea is not a single disease, but only a symptom of different diseases. Diarrhea is characterized by an increase in bowel movements (more than three per day) and an increased liquidity of stools. Acute diarrheas are defined as those that last less than four weeks, whereas chronic diarrheas persist for more than four weeks. The pathophysiological basis of diarrhea is a disturbed enteral water- and electrolyte balance, which can be caused by an increased secretion of osmotically active electrolytes (secretory diarrhea) or the increased ingestion of osmotically active substances (osmotic diarrhea). The stool characteristics allows to distinguish watery, bloody and fatty diarrhea. Acute diarrheas are mostly caused by an infectious agent (viruses, bacteria and parasites), whereas the differential diagnosis of chronic diarrhea is considerably larger and therefore the diagnostic work-up is more complex.