The ability of students to interpret from, represent mathematical problems in, and interact with, a number of differing representations appears to be a crucial part of understanding mathematical concepts. Here we consider the role of representation in student understanding of derivative and present an outline framework for determining its influence. Analysis of questionnaires and interview data from case studies of two students on the basis of the framework revealed qualitative differences in their understanding of derivative and its associated concepts, and suggesting that representational fluency may be a key marker for this difference in thinking. (Contains 1 table.) [For complete proceedings, see ED500859.]