Using data collected through classroom videotaping, student surveys, and the Experience Sampling Method (ESM), the present study explored associations between teacher-provided intellectual challenge, two types of support (instrumental and emotional), and students' momentary academic engagement in high school science classrooms. Results of 3-level Hierarchical Linear Models indicate that researchers' assessments of teacher-provided challenge positively predicted students' momentary reports of engagement in science learning activities. Teachers' provision of instrumental support was also positively associated with student engagement. Contrary to expectations, teacher provision of emotional support was not consistently related to students' reports of engagement. Both instrumental and emotional support interacted with challenge such that teachers' simultaneous provision of challenge and support was associated with additional gains in student engagement. Consistent with these findings, overtly obstructive (non-supportive) teacher behaviors were associated with decreases in student engagement when instruction was challenging. Results are discussed in terms of implications for theory and instructional practice. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]