Mindfulness- and Acceptance-based interventions (MABIs) are being used with increasing frequency with sport participants. Research suggests that such interventions may promote sport performance and impact performance-relevant factors, although the current quality and quantity of research is low. Specifically, questions about intervention engagement (i.e., dosage), potentially impacted by stage of change and attitudes toward sport psychology, need clarification. The present study utilized a multi-method, quasi-experimental longitudinal design with female- and male- identified NCAA collegiate athletes to investigate the effects of an MABI. Specifically, one NCAA Division II team (n = 16) and three NCAA Division III teams (n = 43) from two universities in the Appalachian region engaged in 6-week Mindful Sport Performance Enhancement (MSPE) interventions during their offseason or pre-season training period. Two teams served as the primary treatment group while two others served as the nonrandomized waitlist control group; they were all assessed on measures of flow, psychological distress, mindfulness, emotion regulation difficulties, satisfaction with life, readiness to engage in mindfulness practice, attitudes toward sport psychology, and subjective ratings of performance and enjoyment. Mindfulness dosage was also measured throughout and following the intervention. Compared to controls, initial MSPE participants reported reductions in depressive symptoms and emotion regulation difficulties, and improvements in self-rated sport performance; at 6-week follow-up, initial MSPE participants reported significant increases in mindfulness and sport enjoyment, in addition to sustained reductions in emotion regulation difficulties. Though pre-intervention readiness did not predict changes across the intervention, higher post-intervention readiness classification was associated with improvements in mindfulness, life satisfaction, and sport enjoyment. Higher post-intervention readiness was also associated with significantly more engagement (i.e., dosage), though neither dosage nor attitudes toward sport psychology demonstrated evidence of moderation on the relationship between readiness and outcomes. These data suggest that readiness may impact engagement in MABIs, though it is still unclear whether there are indirect effects of dosage and attitudes toward sport psychology on outcomes in MABIs with athletes. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]