This article examines the impact of a specially designed in-service model on teacher understanding of selected science concepts. The underlying idea of the model is to get teachers to restructure their own understanding of a selected science topic by having them study the structure and evolution of their students' ideas on the same topic. Concepts on topics from the life, earth, and physical sciences served as the content focus and middle school Grades 4–9 served as the context for this study. The in-service experience constituting the main treatment in the study occurred in three distinct phases. In the initial phase, participating teachers interviewed several of their own students to find out what kinds of preconceptions students had about a particular topic. The teachers used concept mapping strategies learned in the in-service to facilitate the interviews. Next the teachers teamed with other teachers with similar topic interests and a science expert to evaluate and explore the scientific merit of the student conceptual frameworks and to develop instructional units, including a summative assessment during a summer workshop. Finally, the student ideas were further evaluated and explored as the teachers taught the topics in their classrooms during the fall term. Concept maps were used to study changes in teacher understanding across the phases of the in-service in a repeated-measures design. Analysis of the maps showed significant growth in the number of valid propositions expressed by teachers between the initial and final mappings in all topic groups. But in half of the groups, this long-term growth was interrupted by a noticeable decline in the number of valid propositions expressed. In addition, analysis of individual teacher maps showed distinctive patterns of initial invalid conceptions being replaced by new invalid conceptions in later mappings. The combination of net growth of valid propositions and the patterns of evolving invalid conceptions is discussed in constructivist terms.