This research investigated how middle school science teachers integrated engineering into their planned science instruction and the extent to which they conceptually linked science to engineering following participation in a professional development program. Video recordings of six teachers' classroom implementation of teacher-designed units were the main data sources (i.e., more than 3000 min). Additionally, the integrated STEM unit plans were used as a secondary source. A deductive analysis of the data showed that the extent and timing of engineering integration and the extent of conceptual connections between science and engineering varied in six teachers' classrooms. All teachers started the instruction with an engineering design challenge. However, only two teachers integrated engineering throughout the unit. Five out of six participants identified opportunities for students to redesign their engineering solutions both in the unit plan and their unit implementation, whereas one teacher gave students a chance to redesign but it was not stated in the unit plan. Regarding science and engineering integration during the unit implementations, two teachers made topical connections, and only one teacher established deep conceptual connections. The findings of the study reported the difficulty of engineering integration both in earth and life science content domains and showed an urgent need for further teacher professional development programs that explicitly focus on how engineering and science are conceptually linked.