In this paper, the author investigates how the working assessment theories and practices of preservice teachers change in the enactment of those theories and practices in a mentored learning environment. Following 38 secondary English teacher candidates across time, the author tracks preservice teacher growth in knowledge about classroom-based assessment and assessment-driven planning. Teacher candidates reported they were strongly influenced by professional dialogue about planning and assessment in both campus classes and mentored field experiences. Although most teacher candidates grew to accept alternative assessments as valuable evidence sources indicating student learning, they recorded concerns that fell into five overlapping categories: designing goals; rubrics, grading and fairness; grading and motivation; validity of assessments; and time required to plan this way.