In assessment, the portfolio has gained significant interest as a tool to monitor and appraise competence development in multiple domains of professional learning. In this book, a developmental use of assessment instruments is advocated, stressing a personalised or self-regulative and learning-oriented deployment of the assessment tool. Portfolio assessment viewed this way can support knowledge productivity of professionals, thus enhancing professional development. By this we mean the construction of knowledge through feedback and dialogue about performances as a recursive loop to inform the professional about accomplishment. Portfolio assessment therefore informs and scaffolds the learner to develop further; it, so to speak, is pioneering development. Since the first introduction in several settings, teaching education, professional preparation, instructional program evaluation, student learning in several domains: nursing, teaching, training, and human resource development, portfolios have been studied extensively. It is challenging to gauge the routes along which the reasons for the interest in portfolios have shifted from one problem to the other. As we interpret this journey of research it went more or less along the following lines during the past 15 years, shifting its perspective as insights grew and demands changed.