The premise that underlies the pre-service science teacher education program at Monash University is the need to focus on the nature of learning in ways that encourage student-teachers to reconsider their conceptions of learning and how this relates to their view of teaching. The purpose of teaching portfolios is to act as a prompt for student-teachers to reconsider these conceptions and as a way of helping them to better articulate their professional knowledge. The Science (Stream 3) student teachers construct a portfolio of teaching strategies, episodes, ideas, etc. that demonstrate how they see their role as science teachers. The portfolio is ungraded, openended and organised as a dynamic assessment task, not just a static end product. This paper reports on student-teachers' understanding of, and approach to portfolios as they come to understand its purpose and value.