This study examines primary grades teachers' instructional decisions in their mathematics classroom during their participation in a year-long professional development program on formative assessment. Teachers participated in 40 h of face-to-face workshops followed by 40 h of classroom-embedded activities that were facilitated in an asynchronous online format. Inductive analysis of teachers' online discussion forum posts and their frequency of using an internet-based formative assessment system indicated significant variance in teachers' use of the formative assessment tool. Some teachers used the assessment system regularly and reported modifying instruction based on the data collected through formative assessment. However, some teachers demonstrated difficulty determining how the assessments aligned to specific mathematics standards, and how the assessment should inform their use of curriculum. Implications for future research include the need to triangulate data from instructional plans, surveys, and classroom observations, while implications for practice include the need to more explicitly support teachers' instructional planning process using specific curricular resources.