Postweld heat treatment produces both mechanical and metallurgical effects in pressure vessel steels that will vary widely with the composition of the steel, its past thermal and mechanical history including welding, the temperature and duration of the postheat, and the heating and cooling rates accompanying postheat. When the majority of welded pressure vessels were fabricated from carbon steels, postweld heat treatment (PWHT) was performed to relieve the locked-up stresses resulting from fabrication, and the process was identified as stress relief. As higher-strength alloy steels were applied to pressure vessels, it was realized that there were other effects involved, some of which were beneficial and others detrimental to the performance of the vessels.This review is an attempt to clarify the present status of PWHT as a fabrication tool and to provide a background for defining the conditions under which postheating is necessary and those in which it may be detrimental.