In the present study, we used a quantitative diary design to investigate within-person fluctuations in student engagement and performance. Specifically, we analyzed the impact of weekly personal and study resources on weekly student engagement, active learning behaviors, and performance. In addition, we investigated whether students high (vs. low) in trait Openness reacted differently to their weekly resources. The sample was composed of 45 first-year psychology students who filled in a questionnaire over 3�weeks; twice per week (N�=�45�נ6�=�270 occasions)—during the days they had tutorial group meetings. The tutors evaluated each student’s active learning behaviors during these meetings. Results of hierarchical linear modeling analyses showed that study engagement fully mediated the relationship between personal resources and observed learning activities; study resources were indirectly positively related to learning activities through study engagement. In addition, observed learning activities were positively related to the course grade. As hypothesized, trait Openness strengthened the positive relationship between personal/study resources and study engagement. Our findings highlight the importance of fostering students’ engagement. We conclude with a discussion of the theoretical implications for education and suggestions for future research.