We report on a study with 23 PSTs preparing to teach grades 1-8 who were engaged in analyzing a series of student-generated arguments for evidence of student mathematical reasoning (MR). We examined PSTs’ assessment of student MR prior to and after instruction designed to support PSTs’ understanding of how expert-like reasoning in elementary mathematics classrooms might look like. Prior to the intervention, PSTs interpreted and assessed students’ MR looking for evidence of discrete reasoning actions (ex. adapting, exemplifying, representing). After the intervention, rather than assessing student reasoning in terms of the presence or absence of specific reasoning actions, PSTs analyzed the identified reasoning actions on the continuum from less to more expert-like. We discuss the intervention and the specific nature of PSTs’ assessment practices before and after class activities.