The use of a reference temperature to define the transition from ductile-to-brittle behavior is a well-established practice in reactor pressure vessel integrity analysis. Current procedures for analysis of irradiated pressure vessel steels use an Adjusted Reference Temperature (ART or RTPTS) that shift the unirradiated RTNDT value based on Charpy data and apply additional margins. The recent adoption of an ASTM Standard Test Method for Determination of Reference Temperature, To, for Ferritic Steels in the Transition Range (E 1921) has made direct determinations of fracture toughness reference temperature in irradiated materials using Master Curve feasible. ASME Code Case N629 recognizes the potential of this new technology as an alternative means of determining an ART value for use in reactor pressure vessel integrity analysis. This alternative approach has been used in recent plant specific submittals to the NRC. ASTM sub-committee E10.02 is currently considering adopting a standard to provide a unified definition of the Adjusted Reference Temperature. This unified definition would provide consistent procedures for using either RTNDT or Master Curve based definitions of ART. This unified definition would provide common margins for the two approaches and assure a consistent level of confidence in the analysis. The statistical basis of the Master Curve makes it possible to establish a more rational basis for the margins applied in both approaches.