The authors of this report build on past studies by using survey data from a nationally representative sample to examine how middle and high school English language arts (ELA) and mathematics teachers use and perceive their instructional materials in terms of engagement, challenge, and usability. In addition, the authors use interview data to understand teachers' perceptions about what makes instructional materials engaging, appropriately challenging, and usable. "Engagement" is the extent to which the instructional materials pique and sustain student interest and attention; "appropriately challenging" is the extent to which the materials address the academic and learning needs of students; and "usable" is the extent to which the materials feature components that teachers desire and that are easy to enact or adapt to meet the needs of their students. Gaining insight into teachers' perceptions of their materials is important because this insight can affect the work of curriculum developers; it can influence state, school district, and other decisionmakers in their materials-adoption processes. Furthermore, teachers' modifications of materials can affect students' opportunities to achieve academic standards. This insight has become even more important during the coronavirus disease 2019 (COVID-19) pandemic because of instruction moving online and teachers having to adapt their instruction and materials accordingly.