This report describes the longitudinal development of one student, Drew, in terms of his unit concepts in area and volume measurement situations across Grades 2 through 5. Data were collected during individual interviews and an open-response assessment within the context of a four-year longitudinal teaching experiment. Results indicate that Drew's use of unit developed over time with respect to identifying and operating on composite units. In addition, his understanding of the conceptual link between the iteration of a composite unit and the application of multiplication in both area and volume measurement contexts developed gradually. Furthermore, changes in his use of unit in an area measurement context often precipitated changes in his use of unit in a volume measurement context. [For the complete proceedings, see ED584443.]