This investigation explores the critical reflection of two urban high school science teachers as they participate in cogenerative dialogues--weekly discussions with focus groups of students that are held outside of instructional hours and that center on identifying and addressing problem areas of classroom teaching and learning. The study finds that, over their semester-long participation in the dialogues, the teachers often grappled with what they termed the "big questions" of teaching--tensions centering on the extent of scaffolding versus the demands of rigor, district-mandated curriculum versus student-centered inquiry, and the competing purposes of collaborative student work. Addressing such tensions within cogenerative dialogues helped the teachers progress from technical and comparative considerations of instruction, to more critical forms of reflection.