It is well known that openings in pressure vessels constitute a major limitation in the use of higher allowable design stresses, and also constitute a potential source of failure for equipment subject to fatigue loading.Recognizing the importance of this problem, the Pressure Vessel Research Committee has, almost from its beginning, supported research in this field, considering the effects of both internal pressure and external piping loads. By 1955, in recognition of the importance of this problem to nuclear and other industries, a Subcommittee was appointed, with F. S. G. Williams as chairman, having as its assignment the development of a theory for reinforced openings.Development of such theory was to be based on present knowledge (as exemplified by classical shell theory) plus additional information obtained from tests.