In the past, openings in pressure vessels have been reinforced according to the area-replacement rule.Vessels so designed have, by and large, performed satisfactorily; however, in the relatively few cases where problems have developed in pressure vessels; those problems have often been associated with the nozzles or openings.Prior to the work of the past few years, the stress levels, fatigue loading resistance, and maximum pressure capacities of nozzles in pressure vessels were known only very roughly.The trend towards the use of higher strength materials and higher nominal stresses in pressure vessel design, along with the advent of nuclear vessels with their very high reliability requirements, intensified the need for better knowledge of the behavior of reinforced openings in pressure vessels.In recognition of this need, the Subcommittee on Reinforced Openings of the Pressure Vessel Research Committee has sponsored a broad program of both experimental and analytical work directed toward the determination of the behavior of reinforced openings in pressure vessels, considering both the effects of internal pressure and external loads.A summary of this work, as of the time of publication, is given by J.L. Mershon.