Patients with ulcerative colitis and familial adenomatous polyposis (FAP) are the two largest groups of patients undergoing restorative total proctocolectomy with an ileal pouch-anal anastomosis. These two groups of patients suffer from severe manifestations of two constitutional illnesses. Ulcerative colitis is an inherent predisposition to abnormal inflammation that mainly affects the colon but can also affect other organs. FAP is an inherent predisposition to disordered cell growth due to a constitutional pathogenic variant in a key tumor suppressor gene that mainly affects the colon but can also affect other organs. Patients with ulcerative colitis need proctocolectomy because the inflammation cannot be controlled by medical therapy, because the inflammation is severe enough to endanger the patient’s life, or because chronic inflammation has led to dysplasia and a risk of cancer. Patients with familial adenomatous polyposis need proctocolectomy because the cancer risk associated with the number of polyps and the degree of dysplasia affecting the colon and rectum cannot be controlled by either medications or endoscopy. The most severely affected patients with each of these diseases select themselves to have total proctocolectomy and ileal pouch. In this chapter the implications of making an ideal pouch in each of these conditions will be considered.