This study explored associations between students' perceptions of challenge, teacher-provided support and obstruction, and students' momentary academic engagement in high school science classrooms. Instrumental and emotional dimensions of support and obstruction were examined separately, and analyses tested whether the relationship between challenge and engagement was moderated by teacher support, teacher obstruction, and individual characteristics like gender and grade level. Students' perceptions of challenge were positively related to their momentary reports of engagement in science learning activities, while teachers' instrumental support was positively associated with engagement across all levels of perceived challenge. Even though teachers' provision of emotional support was not predictive of student engagement, teachers' emotional obstruction was negatively associated with student engagement. Teachers' instrumental obstruction had less consistent associations with student engagement, and was only associated with declines in engagement during those moments when students perceived greater challenge in class. Both gender and grade level emerged as moderators of the relationship between challenge and engagement. Results are discussed in terms of implications for future research and instructional practice.