This article is an analysis of the role of assessment portfolios in teacher learning. Over 18 months, 23 science teachers developed, implemented, and evaluated assessments to track student learning, supported by portfolio tasks and resources, grade-level colleagues, and team facilitators. Evidence of teacher learning included (a) portfolios of a sample of 10 teachers and (b) the cohort's self-reports in surveys and focus groups. Teachers gained understanding of assessment planning, tasks and scoring guides, methods of analyzing patterns and trends, and use of evidence to guide instruction. Teachers made uneven progress with technical aspects of assessment and curriculum-specific assessment. Research is needed on ways to integrate the benefits of a generic portfolio with strategies to strengthen specific areas of assessment expertise. (Contains 5 tables, 2 figures and 10 footnotes.)