This paper describes performance-based assessment of beginning science teachers who are seeking provisional teaching certificates in Connecticut. The assessment is part of the Science Education Support and Assessment Program (SESAP). The project attempts to turn a summative licensing assessment into a formative instrument to improve science teaching and learning. The SESAP program is based on a model of beginning teacher growth that considers first-year teachers as learners in a development stage; second-year teachers as learners and researchers in an inquiry stage; and third-year teachers as learners, researchers, and professionals. Assessment is regarded as an opportunity to learn, and the system is designed to give teachers an opportunity to present their teaching and reflect on it with a mentor. In the first year teachers are invited to professional meetings in which they learn how to document and analyze their teaching. This leads teachers to the development of teaching portfolios that are scored and evaluated around specific dimensions of teacher knowledge. Four appendixes describe the SESAP system, guidelines for portfolio construction, dimensions of knowledge for teacher assessment, and professional science teaching standards. (SLD)