Megacolon is a state of pathological dilatation of the colon without mechanical causes, including malignancy, volvulus, and strangulation. It is classified as “acute” or “chronic” based on clinical course. Acute megacolon includes acute colonic pseudo-obstruction and toxic megacolon. Chronic megacolon is further classified as “idiopathic” or “secondary” based on the absence or presence of underlying causes. Treatment and prognosis vary depending on the pathology. For acute colonic pseudo-obstruction, neostigmine administration and endoscopic decompression are effective. If the small intestine is not involved, total colectomy may be successful. It is generally transient with a relatively good prognosis. In toxic megacolon, surgical treatment is absolutely indicated in cases of perforation, uncontrollable bleeding, or organ failure. Even if surgical treatment is not indicated, bowel rest and decompression are necessary with systemic monitoring in preparation for potential surgical intervention. Prognosis is generally poor, with a mortality rate of 19–45% in cases with ulcerative colitis, and 38–80% in those with pseudomembranous colitis. For chronic megacolon, dietary and pharmacological therapies are fundamental. Surgical treatment is indicated for cases with insufficient response to conservative therapies or recurrent sigmoid volvulus. While refractory, prognosis is good if defecation control is achieved or surgical treatment is successful.