This report summarizes the methods and bases used by the Task Group for Piping Flaw Evaluation to develop allowable flaw sizes for the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section XI, IWB-3640, “Evaluation Procedures and Acceptance Criteria for Austenitic Piping.” Because the Task Group identified two flaw-related failure mechanisms for light water reactor (LWR) austenitic steel piping, two sets of allowable flaw size tables were developed. One set is presented for base metal and nonflux welds, which have high flaw tolerance; a second set is provided for flux welds that can have lower relative toughness and flaw tolerance. Application of the flaw evaluation procedures for cast austenitic piping requires material classification on a case by case basis. Allowable flaw sizes are presented in tabular form, with the allowable part-through flaw depth defined as a function of load and flaw length. Tables are presented for both longitudinal and circumferential flaws, and normal/upset and emergency/faulted conditions. This report also recommends crack growth rate and weld residual stress data needed to perform subcritical crack growth analyses in accordance with Section XI, Appendix C, “Evaluation of Flaws in Austenitic Piping.” These data and the allowable flaw size tables provide the means to determine the acceptabiality for continued service of austenitic piping containing flaws whose sizes exceed the standards specified in Section XI, IWB-3514.3, “Allowable Indication Standards for Austenitic Piping.”