Classroom assessment and the use of student performance data to inform instructional decisions have significant potential to help students meet the learning goals of science education. Research has shown that process-oriented assessment practices are challenging and sometimes ignored aspects of teaching, partly because teachers' conceptions of assessment practice do not reflect a cycle of assessment that continually informs instruction. This study explores middle school science teachers' conceptions of assessment practice based on drawn conceptual models and interviews that were gathered as part of a year-long professional development intervention. Results indicate that participants initially conceived of assessment practice in terms of tangible elements. Changes were seen, however, across the PD as several teachers developed conceptions that were more iterative, in which frequent assessment was used to inform future instruction. These findings raise important questions for how PD can most effectively support teachers' adoption of research-based conceptions of quality science assessment practices.