This paper presents results from a number of trials of a new approach in assessing student conceptions in physics and changes in these conceptions over time. The goal was to explore the potential of Personal Construct Psychology and its central tool, Repertory Grid Technique, to aid in the diagnosis of learner difficulties and eventually the prescription of instructional interventions in science education. The Repertory Grid traces a portrait of the set of an individual's constructs with respect to a given set of elements. It can be used in a computerized form in which an individual identifies distinguishing elements of constructs. The feasibility of the technique in studying physics was evaluated with three subjects of varying expertise (a college student, a college teacher, and a doctoral student in physics). This preliminary study led to trying the technique with a high school student who assembled five grids to reveal concept understanding. A second trial with seven students also supported the usefulness of the approach in revealing deficits in students' conceptual understandings. (Contains 7 grid figures and 26 references.) (SLD)