Diarrhea is the hallmark clinical sign of equine colitis (inflammation of the colon), a condition which can occur in horses of any breed, gender, and age. Horses are particularly susceptible to acute, severe, and sometimes fatal diarrheal illness because of their large colon and caecum. Considerable progress has been made in the last decade in understanding what used to be called “colitis X.” However, many cases of colitis have no known cause. Therefore, in a large proportion of equine cases, the cause of the diarrhea cannot be established. These cases are usually classified as undifferentiated, undetermined, or idiopathic colitis, which is a diagnosis of exclusion made once other known causes of colitis have been ruled out.