In this paper, we explore how candidates in a graduate-level science teacher preparation cohort attended to the substance of student thinking while watching classroom videos or reviewing students' written work. We shared eight case samples of secondary science classroom work with the teacher candidates (six videos and two collections of student work), and asked them to discuss what they noticed in students' ideas and reasoning. We transcribed these conversations and coded candidates' comments in terms of what the candidates were attending to (e.g., student thinking, teachers' actions, student engagement). Our findings show that the teacher candidates were able to attend to specific student ideas and reasoning from the beginning of their pre-service preparation, but their practices of attending become more sophisticated over time. We discuss differences in the ways individual candidates participated and the ways in which participation dynamics within the cohort developed over time as participants assumed different roles and began to regulate their own discussions. We discuss the implications of this work for science teacher education. (Contains 4 tables and 3 footnotes.)