Boiler tubes operating at high temperatures under significant pressure are vulnerable to stress rupture failures. This chapter examines the cause, effect, and appearance of such failures. It discusses the conditions and mechanisms that either lead to or are associated with stress rupture, including overheating, high-temperature creep, graphitization, and dissimilar metal welds. It explains how to determine which mechanisms are in play by interpreting fracture patterns and microstructural details. It also describes the investigation of several carbon and low-alloy steel tubes that failed due to stress rupture.