Hackett, DA, Cobley, SP, and Halaki, M. Estimation of repetitions to failure for monitoring resistance exercise intensity: Building a case for application. J Strength Cond Res 32(5): 1352-1359, 2018-The purpose of this study was to (a) examine the accuracy of Estimated Repetitions to Failure (ERF) during resistance exercise between 2 sessions and (b) compare ERF to rating of perceived exertion (RPE) for determining proximity to momentary failure. Forty-eight adults with recreational resistance training experience performed 3 sets of 10 repetitions at 70% one-repetition maximum (1RM) and 80% 1RM for the chest press and leg press, respectively. At the completion of each set, participants reported their ERF and then continued repetitions to failure to determine actual repetitions to failure (ARF). Two sessions of the same experimental protocol were performed with 48 hours between bouts. For session 1, error in ERF was greater during the first sets compared with third sets for the chest press (2.0 vs. 0.6 repetitions and p < 0.001) and leg press (3.1 vs. 1.6 repetitions and p < 0.001). No differences for error in ERF were observed between sessions 1 and 2 for the chest press (p > 0.944); however, less error in ERF was found for the leg press during set 1 of session 2 (3.1 vs. 1.9 repetitions and p < 0.013). Strong to very strong relationships were found between ERF and ARF (r = 0.59-0.87 and p < 0.01), whereas most relationships for RPE and ARF were small to moderate (r = 0.32 to -0.42 and p < 0.01). Improvement in the accuracy of ERF after a single training bout is minimal, whereas ERF compared with RPE seems to have greater sensitivity for discriminating momentary failure.