OBJECTIVE: Mindfulness interventions have been shown to reduce stress; however, the mechanisms driving stress resilience effects are not known. Mindfulness interventions aim to teach individuals how to: (a) use attention to monitor present moment experiences; with (b) an attitude of acceptance and equanimity. A randomized controlled dismantling trial (RCT) was conducted to test the prediction that the removal of acceptance skills training would eliminate stress-reduction benefits of a mindfulness intervention. METHOD: This preregistered RCT randomly assigned stressed community adults to 1 of 3 conditions: (a) Monitor and Accept (MA) mindfulness training, a standard 8-week Mindfulness-Based Stress Reduction (MBSR) intervention that provided explicit instruction in developing both monitoring and acceptance skills; (b) Monitor Only (MO) mindfulness training, a well-matched 8-week MBSR intervention that taught monitoring skills only; or (c) No Treatment (NT) control. Stress and nonjudgment were measured using ecological momentary assessment (EMA) for 3 days at baseline and 3 days at postintervention. RESULTS: Consistent with predictions, MA participants increased in nonjudgment and decreased in both stress ratings and the proportion of assessments that they reported experiencing feelings of stress in daily life, relative to both MO and NT participants. CONCLUSIONS: This RCT provides one of the first experimental tests of the mechanisms linking mindfulness interventions with stress resilience. These findings suggest that acceptance skills training may be a necessary active ingredient and support the value of integrating acceptance skills training into stress-reduction interventions. (PsycINFO Database Record (c) 2019 APA, all rights reserved).