This report is an analysis of the role of assessment portfolios in teacher learning. Over 18 months, 19 experienced science teachers worked in grade-level teams to design, implement, and evaluate assessments to track student learning throughout a curriculum unit, supported by semi-structured tasks and resources in assessment portfolios. Teachers had the opportunity to complete three assessment portfolios for two or three curriculum units. Evidence of teacher learning included (a) changes over time in the contents of 10 teachers' portfolios spanning Grades 1-9 and (b) the full cohort's self-reported learning in surveys and focus groups. Findings revealed that Academy teachers developed greater understanding of assessment planning, quality assessments and scoring guides, strategies for analysis of student understanding, and use of evidence to guide instruction. Evidence of broad impact on teacher learning was balanced by evidence of uneven growth, particularly with more advanced assessment concepts such as reliability and fairness as well as curriculum-specific methods for developing and using assessments and scoring guides. The findings point to a need for further research on ways to balance general approaches to professional development with content specific strategies to deepen teacher skill and knowledge. (Contains 10 tables, 2 figures, and 10 footnotes.)