This paper offers an analysis of how six experienced teachers, and two in particular, used portfolios to aid and chart steps in their own professional development. The key finding of the study was that the pattern of growth of professional knowledge conformed strikingly to the central features of the model proposed by the philosopher of science, Karl Popper, to account for the growth of scientific knowledge. That is, the use of portfolios assisted these teachers in identifying and formulating problems, in proposing tentative theories for their solution, in testing those theories against their experience of implementation, and in moving on to new problems of practice. The research therefore supports a Popperian structure for teacher professional development that coheres with a conception of teachers as self-learners, or autonomous learners. (Contains 3 tables.)