Despite the clear benefits of physical activity on recovery and survival following an acute coronary syndrome (ACS), most of the 1 million Americans suffering an ACS each year are unable to reach adequate levels of activity. Comprehensive in-person programs to promote activity after an ACS can be effective, but they are attended by relatively few patients. Telephone-delivered motivational interviewing (MI), which has led to increased physical activity in cardiac patients, is a promising evidence-based alternative to in-person programs. However, the overall magnitude of MI’s effects on physical activity has been relatively modest in cardiac-related populations, and additional content may be needed. Psychological well-being (e.g., optimism, positive affect) is prospectively and independently associated with greater physical activity, and therefore cultivating well-being in post-ACS patients could provide an additional method to promote activity in this population. Positive psychology (PP) interventions utilize specific activities (e.g., reflecting on positive events, using personal strengths, imagining a better future) to increase well-being, and they have led to increased physical activity in patients with m edical illness. A combined PP-MI intervention could be a powerful, synergistic approach to promoting activity after an ACS. PP could increase physical activity by improving well-being, while participants set and achieve personalized activity goals. PP could also boost participant engagement in M I could enhance motivation and help M I by fostering optimism and self-efficacy, both of which are linked to greater uptake of interventions like MI. In an NHLBI-funded multiphase project, our team has developed and piloted a new, remotely-delivered PP-MI program in post-ACS patients, and PP-MI led to substantial increases in physical activity in this initial work. Accordingly, we now propose a randomized controlled trial of this 24-week phone- and text-based PP- MI intervention, compared to contemporary treatment as usual, among 280 post-ACS patients with low baseline physical activity. PP-MI utilizes weekly phone sessions for 12 weeks and twice-weekly text messaging for a total of 24 weeks to provide ongoing content and support. We will conduct follow-up assessments at 12, 24, and 48 weeks. The primary outcome for this trial is improvement in moderate to vigorous physical activity (MVPA, measured via accelerometer), and the primary study hypothesis is that PP-MI will be associated with significantly greater improvement in MVPA than treatment as usual at 24 weeks. We will also examine the effects of PP-MI on multiple additional outcomes, including other measures of activity (light activity, sedentary time), psychological/functional measures, and health-related outcomes, including cardiac biomarkers and rates of major adverse cardiac events. Finally, we will systematically assess potential clinical implementation of the PP-MI intervention. If this new...