This guide is intended to inform medical teachers about the use of portfolios for student assessment. It provides a background to the topic, reviews the range of assessment purposes for which portfolios have been used, identifies possible portfolio contents and outlines the advantages of portfolio assessment with particular focus on assessing professionalism. The experience of one medical school, the University of Dundee, is presented as a case study. The current state of understanding of the technical, psychometric issues relating to portfolio assessment is clarified. The final part of the paper provides a practical guide for those wishing to design and implement portfolio assessment in their own institutions. Five steps in the portfolio assessment process are identified: documentation, reflection, evaluation, defence and decision. It is concluded that portfolio assessment is an important addition to the assessor's toolkit. Reasons for using portfolios for assessment purposes include the impact that they have in driving student learning and their ability to measure outcomes such as professionalism that are difficult to assess using traditional methods.