This paper presents results from two qualitative studies of preservice and inservice teacher portfolios that considered how portfolios function as tools for knowledge representation and reflective thinking. In the first study, data were collected from think-aloud protocols, semi-structured participant interviews, and the analysis of supporting Teacher Education Program portfolio documents and the completed portfolios. Participants were in the final term of a Master's in Teaching program. In the second study, data were obtained through semistructured interviews and ethnographic observations of four accomplished secondary school teachers. These studies suggest the contextual factors that may determine portfolio effectiveness and shed light on the dilemmas faced when the attempt is made to translate the theory of teacher portfolios into practice. The dilemmas revolve around the following questions: (1) superficial description or deep reflection? (2) dead-ends or continuing professional development? (3) high-stakes assessments--motivating or distorting? (4) new technological tools--affording or constraining? and (5) portfolios and professional status--recognition or risk? (Contains 10 references.) (SLD)