Context: Although many studies have looked at the teaching of mathematics or at the teaching of reading, few have looked at teaching in both subjects and attempted to discuss characteristics of quality instruction across these two domains. In this article, we reflect on our attempts to do so. We focus on one aspect of instruction, the extent of cognitive demand that characterizes reading and mathematics instruction in fourth- and fifth-grade classrooms. Purpose: We pursued this work to share the difficulties faced in working across the two subject areas and how the differences and similarities in the two subject areas influenced data collection and the interpretation of results. We also wanted to explore whether the instructional styles of teachers who teach in both subject areas exhibited similar amounts of cognitive demand. Research Design: After discussing the struggles faced in data collection, terminology used, and the comparability of research bases in the two domains, we report on an exploratory analysis of classroom observation data from the High-Quality Teaching study. Three aspects of cognitive demand were assessed through the classroom observations of approximately 550 lessons in reading and 600 lessons in mathematics: demand of tasks posed by the teacher, demand of students' responses, and demand of the lesson content. Using data related to these aspects of cognitive demand, we also compared the level of cognitive demand in the reading and mathematics classes of the 69 teachers who taught both subjects. Conclusions: Our findings suggest that the level of cognitive demand exhibited in the tasks teachers pose and the responses and work of students are similar in mathematics and reading. However, we found that the cognitive demand associated with content was higher in reading than in mathematics. For teachers who teach both reading and mathematics, only a small percent demonstrated the same demanding instruction for tasks, responses, or content regardless of subject area. Are the differences real, or an artifact of the definitions and protocols used to compare the subject areas? In either case, we raise issues related to teacher education, professional development, and evaluation. Should teacher policies incorporate information about potential differences in quality instruction across disciplines, and, if so, how? If quality instruction across subjects differs, what are implications for policies that shape expectations for teachers who teach both reading and mathematics?