Diarrhea is a clinical symptom characterized by an increase in the volume, wateriness, or frequency of bowel movements. Diarrhea can be osmotic, secretory, malabsorptive, exudative, due to abnormal intestinal motility, or due to bacterial overgrowth. In osmotic diarrhea, for example, the gut mucosa fails to absorb intestinal contents into the blood stream resulting in accumulation of these substances in intestinal lumen. Subsequently, excessive amount of water remains in stool leading to diarrhea. This book presents research on various types of diarrhea, as well as their causes and the available treatments.