The emphasis of reform-oriented science education today focuses on engineering integration in K-12 science classrooms. However, there is little research, particularly longitudinal research, on how different approaches to engineering integration influence student learning and interest. To address this gap in the literature, this study analyzed a middle school life science teacher's enactment of three design-focused life science units and student performances over a 3-year period. Findings indicate that the design and enactment of each unit reflects a unique engineering integration approach: add on, implicit, and explicit. Moreover, the ways the teacher talked about engineering varied among each curriculum unit. Through the analyses of 330 students' pre- and post-content tests and interest surveys as well as videotaped classroom instruction, we found that explicit engineering integration and engineering language use in classroom instruction resulted in higher student learning gains in science and engineering, but they did not have significant effects on students' interest in science and engineering. We explore the implications for curricular materials and discuss a need for long-term professional development and support for teachers.