Youth physical activity and fitness have been reported to influence cognition and academic related outcomes. Despite the potential benefits of muscular fitness, few intervention studies have examined the impact of an intervention that has incorporated both cardiorespiratory and muscular fitness training on cognition and academic performance in children. Previous studies have mainly been implemented either during the school day or immediately after-school. Although recess may be an ideal time to promote physical activity because it does not compete with other academic demands, it has been an understudied setting. Therefore, the purpose of this dissertation project was to examine the feasibility, acceptability, and the preliminary efficacy of a 3-month recess-based combined fitness (i.e., cardiorespiratory and muscular fitness) program on cognition and academic performance in elementary school-age children. Two elementary schools were randomly assigned to either the combined fitness intervention or control group. The intervention was implemented during recess for 15 minutes/weekday for 13 weeks while the control group continued participating in their regular recess sessions. Process evaluation data (feasibility and acceptability) were collected throughout the intervention. Executive functions, classroom behavior, fitness, and physical activity were collected at baseline and 3-months. Process evaluation data showed that the program achieved high intervention session intensity dosage (mean percentage of maximal heart rate = 58.0±5.8%), number of implemented sessions (88%), and percentage of sessions implemented as planned (78% of sessions). However, intervention session intensity dosage based on accelerometry (% of time spent in moderate-to-vigorous activity: 41.7±14.5%) and participation (19.4% attendance rate) were lower than expected. Moderate-to-vigorous physical activity during recess sessions was significantly higher in the intervention group, compared to the control group (intervention group = 41.7±2.1%; control = 30.4±0.2, P