Issues that must be addressed in designing procedures for aggregating portfolio data are considered. These issues have profound implications for what is aggregated and how data are collected, combined, and interpreted. Portfolio assessment occurs at the intersection of instruction and assessment; it requires students to collect and reflect on examples of their work, providing both an important instructional component to the curriculum and offering an opportunity for complex authentic assessments. A multidimensional, cognitive process model of assessment is recommended in this paper. The cognitive model for assessing portfolios is structured much like the program evaluation model developed by R. Stake (1967). The model is designed to be broadly descriptive, yet to provide a framework for presenting high quantitative and qualitative data in a coherent fashion. Its results are judgments taken from the perspectives of multiple parties. The model does not provide a single score; rather, it provides a comprehensive view of complex learning outcomes in a context where instruction and assessment are inseparable. The three-dimensions of the model are the following: (1) activity, including assessment activities intrinsic to both compiling a portfolio and aggregating across portfolios; (2) historical, including changes beginning from conditions at the outset through transactions to outcomes; and (3) stateholder, which reflects viewpoints of stakeholders with an interest in the portfolios. Three sample portfolio projects are outlined: the Pupil Product Portfolio; a writing portfolio; and a metacognitive letter. Five figures and a 65-item list of references are included. (TJH)