National reform documents are calling for an upsurge in the development of integrated science, technology, engineering, and mathematics (STEM) curricula to simultaneously increase the number of kindergarten through Grade 12 (K-12) students involved in STEM-related coursework and increase the competitiveness of the United States in the STEM-related workforce. With this emphasis on the development of STEM curricula, there is also a need to improve the understanding of education professionals as to what teachers' conceptions of integrated STEM currently are. This study explored how teachers' conceptions of integrated STEM developed over the course of a 3-week-long summer professional development whose focus was on bringing integrated STEM education to science classrooms. Our findings revealed a total of 8 distinct conceptions as represented by teachers' drawn models that shifted in usage over the course of the 3 weeks. Overall, these conceptions started as simple models that grew to more complex models. These findings provide insight into (a) how K-12 science teachers conceptualize STEM integration and (b) how, if at all, these conceptions change over the course of a professional development program. Our findings also indicate the sensitive nature of conceptions of STEM integration that can be influenced by professional development experiences.