Description: Portfolios are increasingly being used in teacher education (Tigelaar, Dolmans, Wolfhagen & Van der Vleuten, 2004). They are designed to promote meaningful and authentic assessment in relevant contexts (e.g., Allal, Wegmuller, Bonaiti-Dugerdil & Cochet Kaeser, 1998) and thereby constitute an "alternative" practice to traditional assessments. Portfolios entail the gathering of information over time across various contexts, in both academic settings and classroom situations. They are frequently accompanied by reflective activities and by interactions in different social formats with peers and supervisors, aimed at the negotiation of meaning and the construction of shared understanding (Vanhulle & Schillings, 2004). Among the different types of portfolio used in teacher education (e.g., Tillema & Smith, 2000), we are interested in those whose purpose is to assess students' progress in the development of their professional skills. This paper will focus on co-assessment interactions between a supervisor and a student (Allal, 1999) during conferences which are instrumented by a "professional development portfolio". In a distributed cognition approach, the portfolio can be conceptualized as a cognitive artefact used to represent and manipulate information when carrying out an activity (e.g., Pea, 1993). It becomes the vehicle through which the person interacts with the social and material environment, in a co-constructive relationship between cognitive processes and environmental resources. Béguin and Rabardel (2000) propose an instrumental approach, integrating the notions of "cognitive artefact" and "schemes of use", that is able to transform both activities and tools. Our research questions are the following: What kinds of mediation does the portfolio offer during the interactions between a supervisor and a student? What use of the portfolio is made by each participant? And for what interpersonal mediation of the assessment of the student's progress? Methodology: Our research was carried out in the context of an "integration seminar" in a program for future primary school teachers. The general purpose of the seminar is to foster articulations between practical training in classrooms and theoretical courses at the university. A professional development portfolio was constituted during the last year of the program. Co-assessment conferences between the supervisor and each student were carried out at the end of the academic year. Our paper presents an in-depth analysis of eight co-assessment conferences. The verbal interactions were fully transcribed and analyzed according to categories defined in a previous study (Mottier Lopez & Wegmuller, 2005). Our qualitative analysis identifies the objects of assessment constructed during the exchanges. It infers the modes of portfolio use and takes systematically into account the portfolio contents that are referred to by the participants. A comparison is made between the documents in the portfolio and the topics of the co-assessment interactions during the conference. Conclusions: The literature frequently mentions the important role of social interactions in the construction of portfolios. In contrast, very few results are reported on the nature and the dynamics of social interactions during conferences that are instrumented by a portfolio. Our research aims at contributing to this development. On the basis of our results, we will show the extent to which co-assessment interactions produce new interpretations and appraisals of students' professional skills in comparison with the self-assessments already expressed by the students in their portfolios. Positive impacts and pitfalls of conferences based on portfolios will be discussed.