Purpose: Using the assessment literacy framework, the purpose of this study was to explore preservice teachers understanding and enactment of the message system during an early field experience. Method: Six PTs employed SE seasons totaling 540 min. Seven qualitative data methods were utilized. Data analysis included deductive and inductive analysis using a thematic approach. Trustworthiness included data triangulation, peer debriefing, negative case analysis, and the maintenance of an audit trail. Results: Three main themes derived from the data included: instructional decisions were driven by the SE model and informal assessment; formal assessment was driven by the structure of SE; and high assessment value but low assessment literacy. Discussion/Conclusion: The SE model drove PTs to use formal assessment, but primarily in an evaluative manner. The PTs lacked assessment literacy, felt pressure to follow the model, did not use assessment to inform teaching, and had concerns for future assessment use.