The purpose of this article is to introduce collective musical cognition as a mode for developing diagnostic critical thinking in music at all levels of the educational continuum--kindergarten through graduate school. It defines and illustrates how collective musical cognition is influenced by relevance, dialogue, and reflection as social and educational practices in the classroom. Relevance, dialogue, and reflection--the three components of collective musical cognition--are critical social practices that transform music education pedagogy, whether practiced in the orchestral rehearsal room, the general music classroom, or the choral and band rehearsal rooms. The prevailing idea is student engagement: student engagement with the teacher; student engagement with fellow students; student engagement with and about music; and student engagement with pedagogical procedures that produce successful outcomes for students and teachers. Collective musical cognition yields an array of learning outcomes that are lasting because the pedagogy mirrors what musicians do. Collective musical cognition is a form of community learning because musical thinking and verbal thinking about music are publicly shared in class. Musical and verbal dialogues that result in collective musical cognition are democratic in nature, as we view dialogue as a catalyst to develop an atmosphere of social equality within the classroom.