Abstract: Analytical methods are needed to account for crack closure due to residual stress (RS) fields. Crack growth data from CX holes at multiple applied stress ratios developed by Analytical Processes/Engineered Solutions, Inc. (AP/ES) and Southwest Research Institute (SwRI) has shown a definite response to applied stress ratio with the data strongly suggesting that crack closure is an active mechanism. Closure is suspected to be one major contributor to discrepancies between predicted and observed crack growth behavior in the presence of RS fields. This presentation reports on the progress made by AP/ES and ESRD in the evaluation of the experimental data and the modeling of closure.