Abstract Introduction The assessment of electronic nicotine delivery systems (ENDS) use poses unique challenges that go beyond established assessment methods for tobacco cigarettes. Recent studies have proposed using ecological momentary assessment (EMA), a method to collect self-reported data on mobile devices, or data passively collected by “smart” Bluetooth enabled ENDS to assess use. The current study sought to compare ENDS use data using EMA and puff counts collected from a smart device. Aims and Methods We recruited 18 young adult ENDS users (age M = 23.33; 44.4% female) from the San Francisco Bay Area. For a total of 30 days, participants completed daily diaries by EMA and used a second-generation smart Bluetooth enabled ENDS that collected puff data. Repeated measures correlations, multilevel regressions, and paired t tests assessed concordance of EMA reports and ENDS data. A subset of four highly compliant participants were selected for sensitivity analyses. Results Among all 18 participants, completion of EMA daily diaries was high (77.4%). The ENDS device collected approximately twice as many puffs per day as participants reported. Compared with self-reported number of sessions and amount of e-liquid used, self-reported puff counts had the highest correlation with device-collected puff counts (rrm = 0.49; p < .001). Correlations between self-reported and device-collected puff counts improved among the subset of four highly compliant participants (rrm = 0.59; p < .001). Conclusions Self-reports potentially underestimate use of ENDS. Puff counts appear to be the best self-reported measure to assess ENDS use compared with number of sessions or liquid volume. Implications The comparison of EMA self-reports and passively collected ENDS device data can inform future efforts to assess ENDS use. Self-reported puff counts are preferable over number of sessions or amount of liquid used, but compared with objective usage data, self-reported puff counts may still underestimate actual use. ENDS use behavior is likely higher than users estimate and report. Future research on improved measures of ENDS use is needed.