Research is described that supports the approach used in the new ASTM International standard for the determination of the mode II delamination toughness of unidirectional polymer matrix composites. Reasons for choosing the end-notched flexure test over other candidate methods are first presented. This is followed by results from a combined numerical and experimental study that resulted in a compliance calibration method being chosen for data reduction, and two follow-up studies that addressed appropriate compliance calibration increments. The development of a method of creating static mode II precracks is described and validation studies are presented. Other considerations that went into finalizing the test standard, including the final determination of span length, range of allowable thicknesses, method of locating the delamination tip after precracking, and load levels to be used during compliance calibration are then discussed. Results from an interlaboratory study are presented, which illustrate that the variation in test results that are observed in the end-notched flexure test using the standardized test method are quite small by historical standards. This indicates that the new standard is appropriate for general use, and provides a welcome complement to the existing ASTM International standards for determination of the mode I and mixed-mode I-II delamination toughness.