The Master Curve and its index temperature To have, over the last three decades, gained more and more acceptance in Codes, Standards, and the regulatory frameworks of different countries. For example, the Japanese Electric Association Code (JEAC) 4206 permits use of To-based index temperatures. In the United States RTTo is directly part of Section XI of the ASME Code while use of To along with extended Master Curve techniques is permitted via Code Case N-830-1. Similarly, To is permitted within both the German and the Swiss regulatory frameworks. One barrier to wider use of Master Curve/ To-based methods is the need to perform fracture toughness (KJc) tests using often limited supplies of irradiated or archive materials to determine To. While recent advances in testing technology permit the use of “mini” compact tension specimens, four of which can be machined from one-half of a broken Charpy specimen, the testing and associated engineering and regulatory analysis still represents an additional cost. Fortunately, a variety of methods exist to estimate To from prior information before any additional KJc testing is conducted. This paper will review these methods, which rely on existing information about the material type, limited but not yet valid KJc data sets, and/or Charpy data that is known through reactor pressure vessel surveillance programs. While not a replacement for direct To testing of plant materials these methods can estimate the likely range of To, which can help determine the materials for which additional testing is likely to produce the most benefit.