Supporting the wide range of readers in secondary disciplinary classrooms can involve a number of helpful scaffolds and instructional routines, and the role of "reading purpose" may be particularly important. Research suggests that reading purposes impact reading processes and outcomes, and also that disciplinary experts have discipline-specific purposes for reading. In this qualitative-dominant, mixed-methods study, five high school classrooms were studied to explore what kind of purposes teachers establish for reading in the disciplines and how students perceive those reading purposes. Teachers' (n = 7) reading purposes and related instruction were explored via observations and interviews, and high school students' (n = 135) perceptions of reading purposes for discipline-specific readings in English, Social Studies, and Science were studied via survey. Each classroom was one case in this collective case study with subsequent quantitative analyses to explore differences in students' perceptions of reading purposes based on their reading levels. Results suggest that teachers created reading purposes aligned with three main goals: fostering general comprehension, building disciplinary topic knowledge, and enacting disciplinary expert habits of reading. Findings also suggest that students articulated purposes for reading generally, but with few connections to the reading practices of disciplinary experts. Chi Square analyses of the student level data showed no significant differences between students' perceptions of reading purposes based on their reading levels. Implications, particularly related to the distinction between reading for authentic disciplinary purposes and reading for "school" purposes, are explored.