Using evidence of student thinking and performance is crucial to enacting ambitious science teaching because intimate, formative data can be responsive to student ideas about science. However, narratives and policies about assessment and data use do not always position teachers to draw on the breadth and variety of evidence from the classroom to make instructional decisions that foster sensemaking. Thus, understanding whether, when, and how teachers view and use assessment and other forms of student data is critical to understanding classroom interactions and learning. This study explores teachers' baseline assessment conceptions and practices as well as their changes with relation to assessment and data use across a year-long intervention using a portfolio of classroom assessment artifacts. Middle school science teachers participated in professional development in which they reflected on assessment artifacts in light of nine dimensions of effective science assessment and data-use practice. Drawing on interviews, as well as baseline and outcome portfolios from the classroom, we noticed that initial conceptions of practice focused on the variety and frequency of assessment. Nine months later, participants' practice showed growth in several dimensions, but differentially among teachers based on their baseline proficiency. Teachers were more likely to show growth in dimensions related to feedback and data use if they had initially showed proficiency in articulating clear learning goals and had previously aligned their assessments to the stated goals. These findings suggest a possible developmental trajectory for improving teachers' assessment and data use practice.