Colonic ischemia is most frequently encountered after major abdominal vascular surgery, when the circulation of a patient’s colon is particularly vulnerable to loss of one of the collateral vessels. This often presents in a postoperative patient in an intensive care setting [1]. The diagnosis may be suggested by a raised WBC or the appearance of blood passed per anum. Diagnosis is best made by colonoscopy. This chapter discusses the endoscopic diagnosis of colonic ischemia.