This study investigated 14 Hong Kong primary school teachers’ conceptions of and concerns about the implementation of integrated STEM education in primary school. Data were collected through concept maps and interviews with the teachers. Results revealed that in addition to representing STEM as simplistic models (e.g., STEM as an acronym), the teachers mostly seemed to model STEM integration conceptually using integrated disciplines, science as context, or real-world problem solving as context. Not many teachers were found to represent STEM using engineering design progress as context. Results also indicated that the teachers expressed intense concerns regarding personal inadequacy of STEM knowledge or skills (personal), student outcomes of STEM education (consequence), and limited instructional time (management). This study suggested providing sustained teacher professional development, creating supportive school environments, and building a collaborative STEM community as strategies to facilitate changes in teachers’ concerns and to advance STEM implementation in primary schools. Copyright © 2022 Koninklijke Brill NV, Leiden.