In this paper, I provide a description of assessment through the use of portfolios. Firstly, I give a brief overview of the principles of formative and reflective, as opposed to summative, evaluation. In line with a metacognitive methodology, the portfolio is presented as a very worthwhile instrument for a self-evaluation process; examples are given from the areas of language learning and reading comprehension. The final part of the paper takes into account possible difficulties in the provision of adequate valuation of learners´ portfolios, and offers some useful evaluation criteria as a response. Some supplementary links to different Web sites are given so as to shed further light on the concept of the portfolio, as well as to provide further information and related material.