Experiential acceptance-an orientation of receptivity and noninterference with present-moment experiences-is described as central to mindfulness interventions, yet little experimental work has tested acceptance as a mechanism for mindfulness intervention effects. Guided by Monitor and Acceptance Theory (MAT), this review situates acceptance as an emotion regulation mechanism and reviews self-report mindfulness literature showing that attention monitoring skills are only associated with beneficial mental and physical health outcomes when accompanied by acceptance skills. New experimental dismantling work shows that removing acceptance training from mindfulness interventions reduces their efficacy for improving stress, positive emotion, and social relationship outcomes. Overall, converging evidence demonstrates that acceptance is a critical emotion regulation mechanism of mindfulness interventions. This work advances basic research, has translational value, and offers opportunities for future research.