This article examines one of the challenges to the integration of classroom and large-scale portfolio assessment, a challenge posed by the use of a student's classroom portfolio for large-scale assessment of his or her individual competencies. When work is composed with the support of peers, teachers, and parents, whose work is being judged? The validity of inferences drawn from the assessment can be compromised unless the question can be answered. Experiences with portfolio assessment in Vermont and in another study conducted by the Center for Research on Evaluation, Standards, and Student Testing (California) suggest that the quality of student work reflects not only a student's competence, but also the amount and quality of support received from others. Procedures that highlight a student's contribution to the work must be developed, complicated though this will be. Nevertheless, large-scale portfolio assessment programs appear to carry significant benefits for instructional reform. (Contains 5 tables and 73 references.) (SLD)